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J2EE Technology in Practice Building Business
Applications with the Java 2 Platform 1st Edition Rick
Cattell Digital Instant Download
Author(s): Rick Cattell, JimInscore, Enterprise Partners, Enterprise Partners
ISBN(s): 9780201746228, 0201746220
Edition: 1st
File Details: PDF, 3.70 MB
Year: 2001
Language: english

Table of Contents
Foreword..............................................................................................................................................................1
Acknowledgments...............................................................................................................................................3
About the Editors................................................................................................................................................5
Chapter 1. A Multi−Vendor Standard for Distributed Enterprise Applications.........................................7
1.1 The Networked Economy..................................................................................................................7
1.2 Why Standardize?..............................................................................................................................7
1.3 Why Standardize on J2EE?
................................................................................................................8
1.4 Why a Standard Based on Java Technologies?
................................................................................10
1.5 Why a Book of Success Stories?.....................................................................................................11
Chapter 2. Overview of the J2EE Technology and Architecture.................................................................13
2.1 The Evolution of Distributed, Multitier Applications.....................................................................13
2.2 J2EE Platform Architecture and Technologies................................................................................17
2.3 Application Configurations Supported by the J2EE Architecture...................................................26
2.4 J2EE Roles.......................................................................................................................................28
2.5 Things to Come................................................................................................................................28
ATG/JCrew.......................................................................................................................................................31
Chapter 3. J.Crew Rebuilds its Web Presence with the ATG Dynamo Suite.............................................33
3.1 Technology Evolution
......................................................................................................................33
3.2 Why J2EE Technology?..................................................................................................................34
3.3 Problem/Opportunity Profile...........................................................................................................35
3.4 Collaboration with Sun Professional Services.................................................................................37
3.5 Solution Analysis.............................................................................................................................38
3.6 Benefits............................................................................................................................................45
3.7 Looking Forward.............................................................................................................................46
BEA/Homeside Lending...................................................................................................................................47
Chapter 4. HomeSide Deploys Electronic Lending on BEA's WebLogic J2EE Server.............................49
4.1 The Project.......................................................................................................................................49
4.2 Business Problem.............................................................................................................................51
4.3 Technology Choices
.........................................................................................................................52
4.4 Vendor Selection
..............................................................................................................................54
4.5 Application Architecture
..................................................................................................................54
4.6 Solution Analysis.............................................................................................................................57
4.7 Current Results................................................................................................................................59
4.8 Future Directions.............................................................................................................................60
4.9 Lessons Learned
...............................................................................................................................62
Borland/AT&T Unisource
................................................................................................................................63
Chapter 5. AT&T Unisource: Cost−Optimized Routing Environment on the Borland AppServer........65
5.1 Technology Adoption......................................................................................................................65
5.2 Business and Technological Challenges..........................................................................................66
5.3 Approaching the Challenges............................................................................................................68
i

Table of Contents
Chapter 5. AT&T Unisource: Cost−Optimized Routing Environment on the Borland AppServer
5.4 The Solution.....................................................................................................................................72
5.5 Life after CORE...............................................................................................................................86
Brokat/Codexa
...................................................................................................................................................89
Chapter 6. Codexa: Building a Big Bang Architecture with Brokat's GemStore J2EE Server................91
6.1 Codexa Big Bang Architecture Explodes onto the Scene...........................................................91
6.2 Charting Galaxies of Financial Information....................................................................................91
6.3 J2EE Helped Codexa Bring Order to Its Universe..........................................................................92
6.4 System Architecture: Layers Upon Layers......................................................................................92
6.5 Application Architecture: Billions and Billions of InfoBytes.........................................................96
6.6 The Working Solution: Codexa in Action.......................................................................................99
6.7 Achieving the Big Bang.................................................................................................................101
6.8 Codexa Through Time...................................................................................................................106
Chapter 7. Java Technology BuildseTapestry.com ASP for Charities with Forte Tools.........................107
7.1 The Project.....................................................................................................................................107
7.2 The Company.................................................................................................................................107
7.3 Technology Adoption....................................................................................................................108
7.4 Opportunity: The Business Problem..............................................................................................108
7.5 The Solution...................................................................................................................................111
7.6 Vendor Selection
............................................................................................................................112
7.7 Application Architecture
................................................................................................................113
7.8 Solution Analysis...........................................................................................................................114
7.9 Future Directions...........................................................................................................................116
7.10 A Rich Tapestry...........................................................................................................................118
Forte/eTapestry...............................................................................................................................................119
Chapter 8. HP Bluestone's Total−e−Server at Altura International: Deploying J2EE for
Performance and Scalability.........................................................................................................................121
8.1 The Company.................................................................................................................................121
8.2 The Challenge................................................................................................................................121
8.4 Altura Merchant Operating System...............................................................................................122
8.5 HP Bluestone Total−e−Server and the J2EE Specification...........................................................128
8.6 Configuring the Altura Merchant Operating System Framework.................................................133
8.7 Benefits of the J2EE Platform and HP Bluestone to Altura..........................................................136
HP Bluestone/Altura.......................................................................................................................................139
Chapter 9. Honeywell and Bekins Succeed with IBM.................................................................................141
9.1 IBM and the Evolution of e−Business...........................................................................................141
9.2 Honeywell......................................................................................................................................143
9.3 Bekins............................................................................................................................................149
IBM
...................................................................................................................................................................161
ii

Table of Contents
Chapter 10. International Data Post Brings Snail Mail to the Internet Age with iPlanet.......................163
10.1 Company Profile..........................................................................................................................163
10.2 Problem/Opportunity Profile: The Applet Dilemma...................................................................167
10.3 Solution Analysis: The Lifecycle of a Hybrid Letter
...................................................................168
10.4 Future of Hybrid Mail..................................................................................................................169
10.5 A Multitiered Architecture
...........................................................................................................170
10.6 A Bounty of Benefits...................................................................................................................172
iPlanet
...............................................................................................................................................................175
Chapter 11. CERN Simplifies Document Handling Using the Oracle Application Server
......................177
11.1 EDH Application.........................................................................................................................177
11.2 The EDH Component Model.......................................................................................................179
11.3 Migration to EJB: First Steps
.......................................................................................................183
11.4 The CERN Material Request.......................................................................................................188
11.5 Deployment Descriptors..............................................................................................................191
11.6 Putting It All Together.................................................................................................................195
11.7 CERN's Experience
......................................................................................................................198
Oracle/CERN
...................................................................................................................................................201
Chapter 12. USMTMC Overhauls Small Package Shipping with SunPS.................................................203
12.1 Global Freight Management, Military Traffic Management Command, Mission
.......................203
12.2 Technology Evolution
..................................................................................................................204
12.3 The Small Package Application...................................................................................................205
SunPS/USMTMC............................................................................................................................................217
Glossary...........................................................................................................................................................219
iii

Foreword
This book is for the skeptics. In 1996, the skeptics thought the Java platform would have inadequate
performance for Internet and intranet servers. But they were proven wrong: Thousands of scalable Java
technology−based servers are now online. In 1997, the skeptics said that Sun's community
consensus−building process could not compete with established standards processes to produce a viable
platform. But it didwith an overwhelming groundswell. In 1998, the skeptics said the J2EE platform would
be too big and complicated to implement, and that Sun would be unable to get others to adopt it. But it was
widely adopted, and the design proved very powerful. In 1999, the skeptics said the J2EE platform would
come out years late, that it would take too long to complete specifications, a reference implementation, and a
compatibility test suite. But the J2EE platform came out right on schedule at the end of the year, with all these
deliverables. In 2000, the skeptics said that vendors wouldn't take the compatibility tests seriously and would
not implement the J2EE platform in their mainstream products. But they did; all the leading vendors became
J2EE licensees, and over twenty vendor products have already passed the extensive J2EE compatibility test
suite. In 2001, the skeptics questioned whether real enterprise applications would be implemented and
deployed successfully on the J2EE platform. But they have been. This book is the proof.
This book is for the optimistsdevelopers, engineering managers, CTOs, CEOs, and others who will have the
foresight to bet their enterprise on a promising state−of−the−art platform that can put them ahead of their
competition. In this book, these people will find examples that will help them design their own solutions, and
case studies to demonstrate to their colleagues that J2EE is a powerful, proven platform. There have been over
a million J2EE platform downloads from Sun since its release a year ago, not to mention thousands of
customers who use J2EE−compatible products from one of the two dozen vendors that have licensed the J2EE
platform to date.
This book is for those who want to better understand the J2EE platform. It demonstrates the most important
feature of the platformthat it is an industry−wide initiative, with support and contributions from many
companies and many people. The J2EE platform is not one product from one company. It's a standard
framework around which the leading enterprise vendors are competing to build innovative,
high−performance, distributed enterprise software platforms. In the pages of this book, you will find
contributions from BEA, IBM, iPlanet, Oracle, and half a dozen other vendors, as well as their customers:
AT&T, Bekins, CERN laboratories, the U.S. Army, and many others.
This book is for all the people who are already involved with the J2EE platform. The success of the platform
is the result of outstanding work and vision from a lot of people. I would personally like to thank those
people. In this book, you will read about the most important of themthe people who took the J2EE platform
into the trenches to solve business problems and reap the benefits of this new technology. Their experience is
enlightening. The book's editors, Rick Cattell and Jim Inscore, are ideally suited to bring these experiences to
you: Jim has managed all the technical writing for the J2EE platform, and Rick was instrumental to the
inception and technical architecture of the J2EE platform. We hope you enjoy reading the book as much as all
these people enjoyed working with this technology.
Patricia Sueltz
Executive Vice President
Software Systems Group
Sun Microsystems, Inc.
May 2001

Acknowledgments
We would like to thank the J2EE licensees for their enthusiasm for this book, and for their spirit of
coopetition around the J2EE Platform. Their customers, of course, made this project possible: They were both
helpful and encouraging. It has been fun working on a platform with energy and such momentum. Many of
these contributors are named as authors of the chapters of this book, but others were just as important in
making it happen. In particular, we would like to thank Vince Hunt from Altura; Dave Nyberg from BEA;
Eric O'Neill, Ralf Dossman, and Rebecca Cavagnari from Borland; Eric Odell, Elizabeth Dimit, and Anita
Osterhaug from Brokat; Mark Herring and Dan Gillaland from Forte; Bob Bickel, Mark Mitchell, and Paige
Farsad from HP Bluestone; Jeff Reser from IBM; Patrick Dorsey and Michelle Skorka Gauthier from iPlanet;
Moe Fardoost from Oracle; Barbara Heffner from Chen PR; Corina Ulescu and Bruce Kerr from Sun; and
Brooke Embry and John Selogy from Navajo Company. Patrick Spencer from Sun Professional Services
deserves particular recognition for his enthusiastic participation in the project and his ability to always come
through with the goods. Ann Betser, Kim Olson, and Ullon Willis have also lent valuable support.
The publishing team deserves credit for getting this book out on time while coordinating over a dozen
contributors. Thanks to Mary Darby and Zana Vartanian from Duarte Design for their support on the graphics.
And of course, we're particularly grateful to the Java Series publishing team: Lisa Friendly, Series Editor from
Sun, and Mike Hendrickson, Julie Dinicola, and Jacquelyn Doucette from Addison−Wesley.
Sun's J2EE platform marketing team were very helpful to us: Rick Saletta, Ralph Galantine, Glen Martin,
Milena Volkova, Cory Kaylor, and Bill Roth. Thanks also to Carla Mott, Elizabeth Blair, Vijay
Ramachandran and Jill Smith. The J2EE management team deserves extra credit for keeping the J2EE project
on trackKaren Tegan, Connie Weiss, Janet Breuer, David Heisser, Kevin Osborn, Jim Driscoll, Vella
Raman, Steve Nahm, Bonnie Kellet, Carla Carlson, Vinay Pai, Kate Stout, Linda Ho, Anita Jindal, Larry
Freeman, Peter Walker, Vivek Nagar, and Tricia Jordan.
Finally, special thanks to Jeff Jackson, director of engineering for J2EE, for supporting our enormous
enterprise edition encylopedia and for understanding that people really do read the manual.

About the Editors
Dr. R. G. G. RICK CATTELL is a distinguished engineer in Java platform software at Sun Microsystems,
and a founding member of the Java Platform Group that produced J2EE. He has worked for 17 years at Sun
Microsystems in senior roles, and for 10 years before that in research at Xerox Palo Alto Research Center
(PARC) and Carnegie−Mellon University. The author of more than 50 technical papers and five books, Cattell
has worked with object technology and database systems since 1980. He is co−creator of JDBC, and was
responsible for forming Sun's Database Engineering Group, whose performance tuning helped to make Sun a
leading database server provider. He led the Cypress database management system effort at Xerox PARC, was
a founder of SQL Access, and was founder and chair of the Object Database Management Group (ODMG).
He authored the world's first monograph on object data management, and has received the Association for
Computing Machinery Outstanding Dissertation Award.
Jim Inscore manages technical publications for the Java 2 Platform, Enterprise Edition, in the Java Platform
Software Group of Sun Microsystems. His roles include overseeing developer documentation, such as the
J2EE Tutorial and J2EE Blueprints, providing developer content for the java.sun.com/j2ee Web site, and
serving as technical editor on the Java Series, Enterprise Edition, from Addison−Wesley. Inscore has been
involved with object−oriented and enterprise−related technologies for more than 15 years, working with
developer documentation for organizations that include Oracle, Ingres, NeXT, Kaleida, and Macromedia.
Prior to that, he spent 10 years writing marketing communications materials for the technical marketplace.

Chapter 1. A Multi−Vendor Standard for Distributed
Enterprise Applications
This book is about business and computing, and about the success of a new standard for business computing
in the networked economy.
At its core, business is about relationships and transactions. Business computing is also about relationships
and transactions. While they may seem distinct, the meanings are complementary. Business relationships are
about customers, vendors, and the products and services they buy or sellthe kind of information that
relationships in a computer database are designed to track. Transactions in business are about the exchange of
monetary value, goods, and services. These are the same processes that transactions on a computer database
must perform with complete integrity.
The point is that these days, business and business computing are inextricably intertwined. As business
evolves, the nature of business computing evolvesand vice versa.
Today, business and business computing are evolving together into a networked economy. This book explores
efforts to deal with that evolution, from both the business side and the computing side. It does so with a focus
on how the Java 2 Platform, Enterprise Edition (J2EE), provides a new standard for supporting the business
and technical needs of companies operating in today's economy.
1.1 The Networked Economy
There are shelves of books that describe the new networked economy, so we won't rehash those here. Simply
put, in the networked economy, the exchange of information is as important as the exchange of goods and
services. Even companies in traditional businesses find they have to develop new techniques for managing,
disseminating, and taking advantage of their information resources. Companies need to network, to reach out
to new customers, to interact more effectively with their suppliers, to engage in alliances with new partners.
This economy is largely propelled by the Internet, but it also takes in other networks, such as wireless
networks of cellular phones and hand−held devices, corporate intranets, and a variety of other networks, local
and wide−area. The networked economy is built on two software entities: data and applications.
Historically, the emphasis of information technology has been data managementthat is, large−scale database
management systems have allowed organizations to gather, analyze, and interpret data for strategic advantage.
In the networked economy, the emphasis of information technology shifts toward applications. Distributed
computer applications are the key to reusing existing data and accessing new data. Applications are the key to
establishing secure and robust links with customers, suppliers, and partners. Thus, the key to competing
effectively is the ability to quickly and efficiently develop and deploy innovative applications as new
opportunities arise.
The Java 2 Platform, Enterprise Edition, is designed to provide a standard for developing and deploying the
applications required to take advantage of the reach of the networked economy.
1.2 Why Standardize?
The simplest answer to this question is that standards expand markets and reduce the friction that impedes
transactions. Standards allow businesses to focus on specific business problems rather than complex technical
problems. They provide a lingua francaa common language that allows any business, anywhere, at any time,
to take part in the market.

Consistent, widely supported standards in enterprise computing are particularly important now that the
Internet plays such a large role in new business development. Many students of the networked economy have
noted a wide−scale move from economies of scale to economies of networks, where each new node adds
value to the whole. In the past, transportation and communications markets have benefited most from network
connections and standards. Today, however, industries across the board are able to tap the benefits of the
network, thanks to standards such as the Internet and the World Wide Web. The more a company can use
standards to effectively connect with its customers, suppliers, partnersand even competitorsthe more
effectively it will be able to participate in the market, and the more competitive it will be in the networked
economy.
There's ample historical precedent for the role of standards in facilitating the growth of markets. For example,
railroads became most effective at moving commercial traffic when they adopted a single gauge across whole
continents. Adoption of wide scale AC power standards enabled a far−reaching power grid and created a
commodity market for electrical goods, from light bulbs to power tools to household appliances. Development
of a single telephone standard enhanced the ability of businesses to operate predictably and reliably both
nationally and globally.
All these examples involved standardizing the underlying technology of the network to facilitate competition
in goods and services delivered or made possible by the network. The standards exist in the medium of
interaction and exchange, not in the specific goods and services exchanged. Standards serve the same purpose
as money: They facilitate exchange by providing an agreed−upon medium of exchange by which to conduct
business. This points out an interesting standards paradox: The more businesses standardize on network
technical standards, the more flexible they can be in pursuing new business opportunities and responding to
new business challenges.
For these reasons, wide−scale adoption of e−business standards helps create a large, diverse market for related
goods and services. This, in turn, makes it easier for customers to solve business problems in a variety of
ways.
1.3 Why Standardize on J2EE?
In one of the books on the networked economy referred to earlier, Kevin Kelly notes, Whenever you need to
make a technical decision, err on the side of choosing the more connected, the more open system, the more
widely linked standard.
[1]
[1]
Kevin Kelly, New Rules for the New Economy, New York, Penguin Putnam, Inc. 1998.
The purpose of the Java 2 Platform, Enterprise Edition, is to standardize development and deployment of
applications required by the networked economy. The J2EE standard has been developed by Sun
Microsystems and a variety of partners, many of whom are represented among the success stories in this book.
In the year−and−a−half since its introduction, the J2EE standard has achieved significant momentum among
vendors of enterprise information technology products. A variety of J2EE licensees have now rolled out
commercial products based on this standard, and a number of their customers have developed and deployed
applications using those products.
J2EE supports a standard model for developing distributed transactional applications. Distributed
applications are those that run on several computer systems at once, generally as tiered or layered processes.
For example, the simplest distributed applications generally have a client tier on a desktop, as well as a server
tier on a separate machine, accessible by multiple clients. More complex distributed applications can be
configured by providing business logic tiers in the middle layers and by adding a database tier on the backend.
Transactional applications are those that involve modifying and updating data from various sources,
operations that must be completed in whole or rolled back in whole (see Figure 1.1).
8 Chapter 1. A Multi−Vendor Standard for Distributed Enterprise Applications

Figure 1.1. A Typical Distributed Transactional Application
The client tier of a distributed application frequently runs on a browser on the user's personal computer.
Clients may also be stand−alone applications or other processes. They may run on other devices, such as
cellular phones or personal digital assistants.
The Web tier usually runs on a centralized server or servers located within a corporate computing center,
which delivers content to various clients at the same time. The Web tier may perform other operations, such as
maintaining state information about each user accessing pages on the server, and accessing other tiers of the
application.
The business logic tier generally comes into play when the Web server needs to access specific behaviors that
apply to the business rules for managing an online business or service. For example, an online bookstore uses
business logic to perform customer checkout operations. These are transactional because the books purchased
must be removed from inventory and the customer's credit card must be billed, in one process. If the card can't
be billed for some reason, the books must be left in inventory; if the books aren't available, the card shouldn't
be billed. Transaction management in the business logic tier makes sure this happens consistently and with
data integrity.
The database provides basic storage and access to the organization's data. For example, the data tier accesses
the database that allows an online shopper to browse through a catalog of offerings on an e−tailer's site. In
many cases, the database management system that enables this may be a legacy systema system whose use
precedes the development of the online application or even the World Wide Web. The data source tier may
consist of several systems, acquired at different times for different purposes, but which can interoperate
thanks to transaction processing and interprocess communications facilities in the business logic tier.
Organizations doing business in the networked economy have been developing distributed transactional
applications like these for some time now, well before the evolution of the J2EE standard. The difference is
Chapter 1. A Multi−Vendor Standard for Distributed Enterprise Applications 9

that previous technologies for developing these applications have generally involved vendor−specific
technologies. When a company buys such a solution, it finds itself in vendor−lock. Investments in application
development, in training and support, and in legacy application code and data all serve to bind the
organization to the vendor of that solution.
While vendor lock may be good for the vendors, it's not always useful to customers. In some ways, it may
even be counter−productive for the vendors themselves.
When all the vendors in the marketplace for a technology offer their own unique solutions, they effectively
divide the marketplace into lots of little pies. Each may have its own customers locked in, but every other
vendor has the sameit's hard for one vendor to sell to another vendor's customers. It's also hard for third
parties to offer useful services, since the economies of scale of each small pie market may discourage the
investment required. It also slows the rate of change in the marketplace and reduces its overall size, because
potential customers who want to avoid vendor lock move into the marketplace cautiously.
The goal of the Java 2 Platform, Enterprise Edition, is to eliminate vendor−lock and create one big pie, a
single market in which every vendor can sell to every customer. Vendors can still differentiate their products
and compete effectively by providing better performance, better tools, or better customer support. And
customers can easily reuse the standards−based resources and skills they acquire using products from the
variety of vendors. What's more, third parties can effectively offer ancillary goods and servicestraining,
support, system configuration, custom applications, custom components, and so on. This further enhances the
customer's ability to efficiently and effectively develop applications. The J2EE marketplace represents the
networked economy at work, where every new nodevendor, customer, third partyenhances the value of all
the others.
By breaking vendor−lock, the J2EE standard creates a larger market than exists in a world of proprietary
systems, in which each vendor's basic marketing strategy is to lock in customers. A larger marketplace pulls in
more players and more types of players, and increases the number and variety of offerings. It allows vendors
to concentrate on their strengths, and improves the quality of the resulting applications, since customers can
choose the solutions focused on their precise needs.
1.4 Why a Standard Based on Java Technologies?
The market for the Java programming language and its related technologies has grown to nearly two million
developers in the years since the Java Development Kit was first released on the Web. Implementations of the
Java programming language are now available on desktop systems, servers and mainframes, and in cell
phones, personal digital assistants, and other devices.
The Java programming language has gradually morphed from an interesting way to animate static Web pages
to a sophisticated platform for producing world−class Web−based applications. While the development of
server−side Java technologies may have seemed highly ambitious at one time, server product vendors have
shown increasing interest in the technology, and application developers have readily adopted each new
server−side Java technology as it was introduced.
Java technology on the server started simply enough with JDBC. This technology allowed clients written in
the Java programming language to access server−side databases using standard Application Programming
Interfaces (APIs). Java Servlets were the first server−specific technology for the Web, designed to replace
Common Gateway Interface (CGI) programs written in a platform−dependent way with a technology that
offered the Write Once, Run Anywhere" capabilities of Java technology.
JavaBeans technology paved the way for a component model based on the Java language. Beans provided
portable, reusable chunks of functionality, with well−defined interfaces that could play together easily with

relatively little additional programming. Java Remote Method Invocation (RMI) enabled applications running
in different processes on different machines to communicate with one another in a way that preserved the
object−oriented paradigm, thus simplifying the development of distributed applications in the Java language.
As the portfolio of Java technologies for supporting enterprise−scale distributed applications grew, so did the
interest in presenting them together as a single platform with a unified programming model.
Enterprise JavaBeans (EJB) was the first technology to present the possibility that all these technologies could
work together in a unified application model. Designed to simplify the development of transactional business
logic, EJB defined a component model in which services such as transaction processing and database access
were managed automatically, thus freeing the component developer to focus on the business model of the
application. The momentum generated by the EJB model ultimately led to the development of the Java 2
Platform, Enterprise Edition, a complete platform for supporting component−based, distributed enterprise
applications.
By providing a component−based solution in which certain services are provided automatically, the J2EE
standard commoditizes expertise. The programming expertise required to create sophisticated multitier
applications is largely built into the platform, as well as into platform−compatible offerings in the areas of
standardized components, automated tools, and other products. This simplifies the programming model,
makes expertise available to all, and enables application developers to focus on application−specific
technologies.
1.5 Why a Book of Success Stories?
First, this book of success stories exists because it can exist. That is, there are a lot of organizations out there
today designing and building applications based on J2EE technologies. This book presents just a handful of
the applications that we're aware of. Many IT departments are now specifying J2EE compatibility as a
requirement in new systems they acquire. A wide range of industry partners are providing J2EE−compatible
products. The J2EE platform is a success.
In terms of information about J2EE, there are already a number of publications available, from Sun
Microsystems, our J2EE partners, and other publishers and Web sites, describing technical aspects of the
J2EE platform in detail.
Sun Microsystems and Java software group provide many resoures. The J2EE platform specification, along
with the EJB, JSP, Servlets, and other related specifications, define the core functionality of J2EE. They are
available at http://java.sun.com/j2ee/specifications. The J2EE SDK, which allows developers to try this new
platform before they buy one of the offerings described in this book, is available at
http://java.sun.com/j2ee/downloads. The Java Tutorial, Enterprise Edition, (available at
http://java.sun.com/j2ee/tutorial) focuses on how to get started developing enterprise applications with J2EE.
The J2EE Blueprints book (Designing Enterprise Applications with J2EE) and the Blueprints Web site
(http://java.sun.com/j2ee/blueprints) discuss design considerations for architecting applications to take best
advantage of the features of J2EE.
The information in this book is different from other resources in a couple of ways. First, it focuses on
real−world applications built using J2EE technology. It looks at specific business applications of the J2EE
platform and discusses why the technology was appropriate for the problem at hand. It describes architectural
configurations that were made possible by J2EE and how they suit certain requirements, such as time to
market, robustness, scalability, and other features required by modern distributed applications. Where
possible, it describes alternative technology choices that may have been considered in the process of
developing the particular system, and explores the reasons why J2EE technology best suited the technical and
business requirements of the customer. It also explores alternate architectures using J2EE that may have been
considered for a particular application, and explains how the design decisions and tradeoffs that resulted in a
Chapter 1. A Multi−Vendor Standard for Distributed Enterprise Applications 11

real application were made. This book has its own Web site (http://java.sun.com/j2ee/inpractice), where you'll
find the customer success stories here plus additional real−world experiences, as adoption of the J2EE
platform continues.
In addition to its focus on the real world, this book's development was very much in keeping with the
community process for evolving J2EE and other Java technologies. Each of the case studies represents a
partnership between Sun J2EE licensees, and their customersthat is, the folks building successful
J2EE−based products, and the folks acquiring those products to solve the problems they face day to day. This
is very much a community effort. The licensees and customers who have participated in the preparation of this
book are all interested in furthering the adoption of the J2EE platform. Each success story in this book
represents a pioneering effort to try the technology and to work with it to solve business problems.
Before looking at the business applications of J2EE, the next chapter focuses on its technology. It takes a
closer look at both the individual technologies in the platform and at the ways they work together to provide a
complete platform for distributed application development.
For more on the J2EE platform, see http://java.sun.com/j2ee. For more on these J2EE case studies, see
http://java.sun.com/j2ee/inpractice.

Chapter 2. Overview of the J2EE Technology and
Architecture
This chapter examines the architecture of the J2EE platform, the technologies behind the platform, and the
types of components it supports. It looks at some typical application configurations that can be implemented
using J2EE, and at the various roles involved in developing and deploying J2EE applications. To keep the
discussion grounded, this chapter also points out general benefits that J2EE architecture and technologies
provide to IT organizations.
There are a lot of technologies, buzzwords, and acronyms encountered repeatedly as you read the case studies
that follow. This chapter should help you with the specifics of the various J2EE application designs presented
in those discussions.
2.1 The Evolution of Distributed, Multitier Applications
Applications in the networked economy tend to be multitier, server−based applications, supporting interaction
among a variety of systems. These applications are distributedthat is, they run on several different devices,
including mainframes for data access on the backend, servers for Web support and transaction monitoring in
the middle tier, and various client devices to give users access to applications. Clients can include thick
clientsstand−alone applications on the desktopand thin clients, such as applications running in a browser on
the desktop, applications running in personal digital assistants, even cell phones and other personal
communications devices. For business−to−business applications, distributed computing involves
peer−to−peer connections among dispersed server systems.
The proliferation of systems and devices and the extension of the services provided by the server have
increased the complexity of designing, developing, and deploying distributed applications. Distributed
applications are increasingly called on to integrate existing infrastructure, including database management
systems, enterprise information systems, and legacy applications and data, and to project these resources into
an evolving environment of diverse clients in diverse locations.
To help you understand the issues involved in developing these applications, here's a look at some typical
multitier application scenarios.
The earliest distributed applications were client−server applications running on time−sharing computing
systems (see Figure 2.1).A mainframe computer containing data and data management software was
connected to a number of terminals, which could be distributed as widely as the technology allowed. The
networks used were slow; the client systems were called dumb terminals for good reason. But these
client−server systems were easy to develop and maintain because all applications lived on the mainframe.
Figure 2.1. Pure Client−Server Application Architecture

With the arrival of high−speed networks and smart PC−based clients with elaborate graphical user interfaces,
applications moved from the mainframe to the desktop. This meant more processing power for each user, but
less control for IT departments. The application−development process was simplified with a variety of visual
tools and other programming aids, but application deployment in this multitiered environment became a
problem with so many desktop machines and configurations (see Figure 2.2).
Figure 2.2. PC−Based Client−Server Application Architecture
14 Chapter 2. Overview of the J2EE Technology and Architecture

Browser−based applications on the Internet or intranets are a variation on this model. A browser running on a
desktop PC provides access to the server. Applications run on Web servers, providing all the business logic
and state maintenance. Using this configuration, applications can provide everything from simple page lookup
and navigation to more complex processes that perform custom operations and maintain state information.
The technologies supporting this application architecture include plug−ins and applets on the client side, and
Common Gateway Interface (CGI) scripts and other mechanisms on the server side. The problem with adding
functionality in this environment is that there is no single standard for clients or servers, and the applications
assembled in this way are hard to develop and maintain.
While the architecture of multier applications has evolved, new capabilities have been added to the mix. A
pure client−server architecture is viable for a tightly controlled environment, with one type of client and one
backend server providing some business logic and access to data. But the real world soon became more
complicated. Eventually, organizations wanted to connect multiple backend systemsfor example, to connect
a warehouse inventory system to a customer billing system. Another example would be companies that merge
and need ways to integrate the computing capabilities they inherit.
These requirements led to the evolution of the middle tier in enterprise computing in the nineties. In this
configuration, the business logic of an application moves onto a centralized, more tightly controlled system.
Transaction monitors in the middle tier are capable of integrating disparate data sources with a single
transaction mechanism. With this technology, traditionally disconnected systems could become connected
(see Figure 2.3).
Figure 2.3. Multitier Application Architecture with Distributed Transactions
Chapter 2. Overview of the J2EE Technology and Architecture 15

In addition to the need to have multiple databases communicating, the need to have multiple applications
interacting soon became an issue. With millions of lines of code and the corresponding development and
debugging time investment in legacy applications, organizations wanted ways to reuse the capabilities of
existing applications, and to get time−proven systems communicating in new ways. Among the solutions
proposed, the CORBA standard achieved success by allowing modules in various programs to communicate
with one another. This helped support a new era in distributed computing (See Figure 2.4).
Figure 2.4. Multitier Application Architecture with Multiple Servers and CORBA Interoperability
16 Chapter 2. Overview of the J2EE Technology and Architecture

Discovering Diverse Content Through
Random Scribd Documents

origin. In attributing our perceptions to a normal causal origin
outside ourselves, we run a certain risk of error, since the origin may
be unusual: there may be reflection or refraction on the way to the
eye, there may be an unusual condition of the eye or optic nerve or
brain. All these considerations give a certain very small probability
that, on a given occasion, there is not such an outside cause as we
suppose. If, however, a number of people concur with us, i.e.
simultaneously have reactions which they attribute to an outside
cause that can be identified with the one we had inferred, then the
probability of error is enormously diminished. This is exactly the
usual case of concurrent testimony. If twelve men, each of whom
lies every other time that he speaks, independently testify that some
event has occurred, the odds in favour of their all speaking the truth
are 4095 to 1. The same sort of argument shows that our public
senses, when confirmed by others, are probably speaking the truth,
except where there are sources of collective illusion such as mirage
or suggestion.
In this respect, however, there is no essential difference between
matters of external observation and matters of self-observation.
Suppose, for example, that, for the first time in your life, you smell
assafœtida. You say to yourself “that is a most unpleasant smell”.
Now unpleasantness is a matter of self-observation. It may be
correlated with physiological conditions which can be observed in
others, but it is certainly not identical with these, since people knew
that things were pleasant and unpleasant before they knew about
the physiological conditions accompanying pleasure and its opposite.
Therefore when you say “that smell is unpleasant” you are noticing
something that does not come into the world of physics as ordinarily
understood. You are, however, a reader of psycho-analysis, and you
have learned that sometimes hate is concealed love and love is
concealed hate. You say to yourself, therefore: “Perhaps I really like
the smell of assafœtida, but am ashamed of liking it”. You therefore
make your friends smell it, with the result that you soon have no
friends. You then try children, and finally chimpanzees. Friends and
children give verbal expression to their disgust: chimpanzees are

expressive, though not verbal. All these facts lead you to state: “The
smell of assafœtida is unpleasant”. Although self-observation is
involved, the result has the same kind of certainty, and the same
kind of objective verification, as if it were one of the facts that form
the empirical basis of physics.
(2) The second proposition, to the effect that the physical
sciences are capable of affording an explanation of all the publicly
observable facts about human behaviour, is one as to which it is
possible to argue endlessly. The plain fact is that we do not yet know
whether it is true or false. There is much to be said in its favour on
general scientific grounds, particularly if it is put forward, not as a
dogma, but as a methodological precept, a recommendation to
scientific investigators as to the direction in which they are to seek
for solution of their problems. But so long as much of human
behaviour remains unexplained in terms of physical laws, we cannot
assert dogmatically that there is no residue which is theoretically
inexplicable by this method. We may say that the trend of science,
so far, seems to render such a view improbable, but to say even so
much is perhaps rash, though, for my part, I should regard it as still
more rash to say that there certainly is such a residue. I propose,
therefore, as a matter of argument, to admit the behaviourist
position on this point, since my objections to behaviourism as an
ultimate philosophy come from quite a different kind of
considerations.
(3) The proposition we are now to examine may be stated as
follows: “All facts that can be known about human beings are known
by the same method by which the facts of physics are known.” This I
hold to be true, but for a reason exactly opposite to that which
influences the behaviourist. I hold that the facts of physics, like
those of psychology, are obtained by what is really self-observation,
although common sense mistakenly supposes that it is observation
of external objects. As we saw in Chapter XIII, your visual, auditory,
and other percepts are all in your head, from the standpoint of
physics. Therefore, when you “see the sun”, it is, strictly speaking,
an event in yourself that you are knowing: the inference to an

external cause is more or less precarious, and is on occasion
mistaken. To revert to the assafœtida: it is by a number of self-
observations that you know that the smell of assafœtida is
unpleasant, and it is by a number of self-observations that you know
that the sun is bright and warm. There is no essential difference
between the two cases. One may say that the data of psychology
are those private facts which are not very directly linked with facts
outside the body, while the data of physics are those private facts
which have a very direct causal connection with facts outside the
body. Thus physics and psychology have the same method; but this
is rather what is commonly taken to be the special method of
psychology than what is regarded as the method of physics. We
differ from the behaviourist in assimilating physical to psychological
method, rather than the opposite.
(4) Is there a source of knowledge such as is believed in by
those who appeal to “introspection”? According to what we have just
been saying, all knowledge rests upon something which might, in a
sense, be called “introspection”. Nevertheless, there may be some
distinction to be discovered. I think myself that the only distinction
of importance is in the degree of correlation with events outside the
body of the observer. Suppose, for example, that a behaviourist is
watching a rat in a maze, and that a friend is standing by. He says to
the friend “Do you see that rat?” If the friend says yes, the
behaviourist is engaged in his normal occupation of observing
physical occurrences. But if the friend says no, the behaviourist
exclaims, “I must give up this boot-legged whiskey”. In that case, if
his horror still permits him to think clearly, he will be obliged to say
that in watching the imaginary rat he was engaged in introspection.
There was certainly something happening, and he could still obtain
knowledge by observing what was happening, provided he abstained
from supposing that it had a cause outside his body. But he cannot,
without outside testimony or some other extraneous information,
distinguish between the “real” rat and the “imaginary” one. Thus in
the case of the “real” rat also, his primary datum ought to be
considered introspective, in spite of the fact that it does not seem

so; for the datum in the case of the “imaginary” rat also does not
seem to be merely introspective.
The real point seems to be this: some events have effects which
radiate all round them, and can therefore produce reactions in a
number of observers; of these, ordinary speech is an illustration. But
other events produce effects which travel linearly, not spherically; of
these, speech into a telephone from a sound-proof telephone box
may serve as an illustration. This can be heard by only one person
beside the speaker; if instead of a speaker we had an instrument at
the mouthpiece, only one person could hear the sound, namely the
person at the other end of the telephone. Events which happen
inside a human body are like the noise in the telephone: they have
effects, in the main, which travel along nerves to the brain, instead
of spreading out in all directions equally. Consequently, a man can
know a great deal about his own body which another man can only
know indirectly. Another man can see the hole in my tooth, but he
cannot feel my toothache. If he infers that I feel toothache, he still
does not have the very same knowledge that I have; he may use the
same words, but the stimulus to his use of them is different from the
stimulus to mine, and I can be acutely aware of the pain which is
the stimulus to my words. In all these ways a man has knowledge
concerning his own body which is obtained differently from the way
in which he obtains knowledge of other bodies. This peculiar
knowledge is, in one sense, “introspective”, though not quite in the
sense that Dr. Watson denies.
(5) We come now to the real crux of the whole matter, namely
to the question: Do we think? This question is very ambiguous, so
long as “thinking” has not been clearly defined. Perhaps we may
state the matter thus: Do we know events in us which would not be
included in an absolutely complete knowledge of physics? I mean by
a complete knowledge of physics a knowledge not only of physical
laws, but also of what we may call geography, i.e. the distribution of
energy throughout space-time. If the question is put in this way, I
think it is quite clear that we do know things not included in physics.
A blind man could know the whole of physics, but he could not know

what things look like to people who can see, nor what is the
difference between red and blue as seen. He could know all about
wave-lengths, but people knew the difference between red and blue
as seen before they knew anything about wave-lengths. The person
who knows physics and can see knows that a certain wave-length
will give him a sensation of red, but this knowledge is not part of
physics. Again, we know what we mean by “pleasant” and
“unpleasant”, and we do not know this any better when we have
discovered that pleasant things have one kind of physiological effect
and unpleasant things have another. If we did not already know
what things are pleasant and what unpleasant, we could never have
discovered this correlation. But the knowledge that certain things are
pleasant and certain others unpleasant is no part of physics.
Finally, we come to imaginations, hallucinations, and dreams. In
all these cases, we may suppose that there is an external stimulus,
but the cerebral part of the causal chain is unusual, so that there is
not in the outside world something connected with what we are
imagining in the same way as in normal perception. Yet in such
cases we can quite clearly know what is happening to us; we can,
for example, often remember our dreams. I think dreams must
count as “thought”, in the sense that they lie outside physics. They
may be accompanied by movements, but knowledge of them is not
knowledge of these movements. Indeed all knowledge as to
movements of matter is inferential, and the knowledge which a
scientific man should take as constituting his primary data is more
like our knowledge of dreams than like our knowledge of the
movements of rats or heavenly bodies. To this extent, I should say,
Descartes is in the right as against Watson. Watson’s position seems
to rest upon naive realism as regards the physical world, but naive
realism is destroyed by what physics itself has to say concerning
physical causation and the antecedents of our perceptions. On these
grounds, I hold that self-observation can and does give us
knowledge which is not part of physics, and that there is no reason
to deny the reality of “thought”.

CHAPTER XVII
IMAGES
In this chapter we shall consider the question of images. As the
reader doubtless knows, one of the battle-cries of behaviourism is
“death to images”. We cannot discuss this question without a good
deal of previous clearing of the ground.
What are “images” as conceived by their supporters? Let us take
this question first in the sense of trying to know some of the
phenomena intended, and only afterwards in the sense of seeking a
formal definition.
In the ordinary sense, we have visual images if we shut our eyes
and call up pictures of scenery or faces we have known; we have
auditory images when we recall a tune without actually humming it;
we have tactual images when we look at a nice piece of fur and
think how pleasant it would be to stroke it. We may ignore other
kinds of images, and concentrate upon these, visual, auditory, and
tactual. There is no doubt that we have such experiences as I have
suggested by the above words; the only question is as to how these
experiences ought to be described. Then we have another set of
experiences, namely dreams, which feel like sensations at the
moment, but do not have the same kind of relation to the external
world as sensations have. Dreams, also, indubitably occur, and again
it is a question of analysis whether we are to say that they contain
“images” or not.
The behaviourist does not admit images, but he equally does not
admit sensations and perceptions. Although he does not say so quite
definitely, he may be taken to maintain that there is nothing but

matter in motion. We cannot, therefore, tackle the question of
images by contrasting them with sensations or perceptions, unless
we have first clearly proved the existence of these latter and defined
their characteristics. Now it will be remembered that in Chapter V we
attempted a behaviourist definition of perception, and decided that
its most essential feature was “sensitivity”. That is to say, if a person
always has a reaction of a certain kind B when he has a certain
spatial relation to an object of a certain kind A, but not otherwise,
then we say that the person is “sensitive” to A. In order to obtain
from this a definition of “perception”, it is necessary to take account
of the law of association; but for the moment we will ignore this
complication, and say that a person “perceives” any feature of his
environment, or of his own body, to which he is sensitive. Now,
however, as a result of the discussion in Chapter XVI, we can include
in his reaction, not only what others can observe, but also what he
alone can observe. This enlarges the known sphere of perception,
practically if not theoretically. But it leaves unchanged the fact that
the essence of perception is a causal relation to a feature of the
environment which, except in astronomy, is approximately
contemporaneous with the perception, though always at least
slightly earlier, owing to the time taken by light and sound to travel
and the interval occupied in transmitting a current along the nerves.
Let us now contrast with this what happens when you sit still
with your eyes shut, calling up pictures of places you have seen
abroad, and perhaps ultimately falling asleep. Dr. Watson, if I
understand him aright, maintains that either there is actual
stimulation of the retina, or your pictures are mere word-pictures,
the words being represented by small actual movements such as
would, if magnified and prolonged, lead to actual pronunciation of
the words. Now if you are in the dark with your eyes shut, there is
no stimulation of the retina from without. It may be that, by
association, the eye can be affected through stimuli to other senses;
we have already had an example in the fact that the pupil can be
taught to contract at a loud noise if this had been frequently
experienced along with a bright light. We cannot, therefore, dismiss

the idea that a stimulus to one sense may, as a result of past events,
have an effect upon the organs of another sense. “Images” might be
definable as effects produced in this way. It may be that, when you
see a picture of Napoleon, there is an effect upon your aural nerves
analogous to that of having the word “Napoleon” pronounced in your
presence, and that that is why, when you see the picture, the word
“Napoleon” comes into your head. And similarly, when you shut your
eyes and call up pictures of foreign scenes, you may actually
pronounce, completely or incipiently, the word “Italy”, and this may,
through association, stimulate the optic nerve in a way more or less
similar to that in which some actual place in Italy stimulated it on
some former occasion. Thence association alone may carry you
along through a series of journeys, until at last, when you fall
asleep, you think you are actually making them at the moment. All
this is quite possible, but so far as I know there is no reason to hold
that it is more than possible, apart from an a priori theory excluding
every other explanation.
What I think is clearly untenable is the view, sometimes urged
by Dr. Watson, that when we are, as we think, seeing imaginary
pictures with the eyes shut, we are really only using such words as
would describe them. It seems to me as certain as anything can be
that, when I visualise, something is happening which is connected
with the sense of sight. For example, I can call up quite clear mental
pictures of the house in which I lived as a child; if I am asked a
question as to the furniture of any of the rooms in that house, I can
answer it by first calling up an image and then looking to see what
the answer is, just as I should look to see in an actual room. It is
quite clear to me that the picture comes first and that words after;
moreover, the words need not come at all. I cannot tell what is
happening in my retina or optic nerve at these moments of
visualisation, but I am quite sure that something is happening which
has a connection with the sense of sight that it does not have with
other senses. And I can say the same of aural and tactual images. If
this belief were inconsistent with anything else that seems to me

equally certain, I might be induced to abandon it. But so far as I can
see, there is no such inconsistency.
It will be remembered that we decided in favour of perceptions
as events distinct from those which they perceive, and only causally
connected with them. There is, therefore, no reason why association
should not work in this region as well as in the region of muscles
and glands; in other words, there is no reason to deny what used to
be called “association of ideas”, in spite of the fact that bodily
changes can also be associated. If a physical basis is wanted, it can
be assumed to exist in the brain. The state of the brain which causes
us to hear the word “Napoleon” may become associated with the
state of brain which causes us to see a picture of Napoleon, and
thus the word and the picture will call each other up. The association
may be in the sense-organs or nerves, but may equally well be in
the brain. So far as I know, there is no conclusive evidence either
way, nor even that the association is not purely “mental.”
When we try to find a definition of the difference between a
sensation and an image, it is natural to look first for intrinsic
differences. But intrinsic differences between ordinary sensations
and ordinary images, for example as to “liveliness”, are found to be
subject to exceptions, and therefore unsuitable for purposes of
definition. Thus we are brought to differences as to causes and
effects.
It is obvious that, in an ordinary case, you perceive a table
because (in some sense) the table is there. That is to say, there is a
causal chain leading backwards from your perception to something
outside your body. This alone, however, is hardly sufficient as a
criterion. Suppose you smell peat smoke and think of Ireland, your
thought can equally be traced to a cause outside your body. The
only real difference is that the outside cause (peat smoke) would not
have had the effect (images of Ireland) upon every normal person,
but only upon such as had smelt peat smoke in Ireland, and not all
of them. That is to say, the normal cerebral apparatus does not
cause the given stimulus to produce the given effect except where

certain previous experiences have occurred. This is a very vital
distinction. Part of what occurs in us under the influence of a
stimulus from without depends upon past experience; part does not.
The former part includes images, the latter consists of pure
sensations. This, however, as we shall see later, is inadequate as a
definition.
Mental occurrences which depend upon past experience are
called “mnemic” occurrences, following Semon. Images are thus to
be included among mnemic occurrences, at least so far as human
experience goes. This, however, does not suffice to define them,
since there are others, e.g. recollections. What further defines them
is their similarity to sensations. This only applies strictly to simple
images; complex ones may occur without a prototype, though all
their parts will have prototypes among sensations. Such, at least, is
Hume’s principle, and on the whole it seems to be true. It must not,
however, be pressed beyond a point. As a rule, an image is more or
less vague, and has a number of similar sensations as its prototypes.
This does not prevent the connection with sensation in general, but
makes it a connection with a number of sensations, not with one
only.
It happens that, when a complex of sensations has occurred at
some time in a person’s experience, the recurrence of part of the
whole tends to produce images of the remaining parts or some of
them. This is association, and has much to do with memory.
It is common to speak of images as “centrally excited”, as
opposed to sensations, which are excited by a stimulus to some
sense organ. In essence this is quite correct, but there is need of
some caution in interpreting the phrase. Sensations also have
proximate causes in the brain; images also may be due to some
excitement of a sense-organ, when they are roused by a sensation
through association. But in such cases there is nothing to explain
their occurrence except the past experience and its effect on the
brain. They will not be aroused by the same stimulus in a person
with similar sense-organs but different past experience. The

connection with past experience is clearly known; it is, however, an
explanatory hypothesis, not directly verifiable in the present state of
knowledge, to suppose that this connection works through an effect
of the past experience on the brain. This hypothesis must be
regarded as doubtful, but it will save circumlocution to adopt it. I
shall therefore not repeat, on each occasion, that we cannot feel
sure it is true. In general, where the causal connection with past
experience is obvious, we call an occurrence “mnemic”, without
implying this or that hypothesis as to the explanation of mnemic
phenomena.
It is perhaps worth while to ask how we know that images are
like the sensations which are their prototypes. The difficulty of this
question arises as follows. Suppose you call up an image of the
Brooklyn Bridge, and you are convinced that it is like what you see
when you look at Brooklyn Bridge. It would seem natural to say that
you know the likeness because you remember Brooklyn Bridge. But
remembering is often held to involve, as an essential element, the
occurrence of an image which is regarded as referring to a
prototype. Unless you can remember without images, it is difficult to
see how you can be sure that images resemble prototypes. I think
that in fact you cannot be sure, unless you can find some indirect
means of comparison. You might, for example, have photographs of
Brooklyn Bridge taken from a given place on two different days, and
find them indistinguishable, showing that Brooklyn Bridge has not
changed in the interval. You might see Brooklyn Bridge on the first
of these days, remember it on the second, and immediately
afterwards look at it. In looking at it, you might find every detail
coming to you with a feeling of expectedness, or you might find
some details coming with a feeling of surprise. In this case you
would say that your image had been wrong as regards the details
which were surprising. Or, again, you might make a picture of
Brooklyn Bridge on paper, from memory, and then compare it with
the original or a photograph. Or you might content yourself by
writing down a description of it in words, and verifying its accuracy
by direct observation. Innumerable methods of this kind can be

devised by which you can test the likeness of an image to its
prototype. The result is that there is often a great likeness, though
seldom complete accuracy. The belief in the likeness of an image to
its prototype is, of course, not generated in this way, but only tested.
The belief exists prior to evidence as to its correctness, like most of
our beliefs. I shall have more to say on this subject in the next
chapter, which will be concerned with memory. But I think enough
has been said to show that it is not unreasonable to regard images
as having a greater or less degree of resemblance to their
prototypes. To claim more is hardly justifiable.
We can now reach a definite conclusion about perception,
sensation, and images. Let us imagine a number of people placed,
as far as possible, in the same environment; we will suppose that
they sit successively in a certain chair in a dark room, in full view of
illuminated pictures of two eminent politicians of opposite parties
whose names are written underneath them. We will suppose that all
of them have normal eyesight. Their reactions will be partly similar,
partly different. If any of these observers are babies too young to
have learnt to focus, they will not see sharp outlines, but a mere
blurr, not from an optical defect, but from a lack of cerebral control
over muscles. In this respect, experience has an effect even upon
what must count as pure sensation. But this difference is really
analogous to the difference between having one’s eyes open and
having them shut; the difference is in the sense-organ, although it
may be due to a difference in the brain. We will therefore assume
that all the spectators know how to adjust the eyes so as to see as
well as possible, and all try to see. We shall then say that, if the
spectators differ as widely as is possible for normal human beings,
what is common to the reactions of all of them is sensation,
provided it is connected with the sense of sight, or, more correctly,
provided it has that quality which we observe to be common and
peculiar to visual objects. But probably all of them, if they are over
three months old, will have tactile images while they see the
pictures. And if they are more than about a year old, they will
interpret them as pictures, which represent three-dimensional

objects; before that age, they may see them as coloured patterns,
not as representations of faces. Most animals, though not all, are
incapable of interpreting pictures as representations. But in an adult
human being this interpretation is not deliberate; it has become
automatic. It is, I think, mainly a question of tactile images: the
images you have in looking at a picture are not those appropriate to
a smooth flat surface, but those appropriate to the object
represented. If the object represented is a large one, there will also
be images of movement—walking round the object, or climbing up
it, or what not. All these are obviously a product of experience, and
therefore do not count as part of the sensation. This influence of
experience is still more obvious when it comes to reading the names
of the politicians, considering whether they are good likenesses, and
feeling what a fine fellow one of them looks and what unmitigated
villainy is stamped upon the features of the other. None of this
counts as sensation, yet it is part of a man’s spontaneous reaction to
an outside stimulus.
It is evidently difficult to avoid a certain artificiality in
distinguishing between the effects of experience and the rest in a
man’s reaction to a stimulus. Perhaps we could tackle the matter in a
slightly different way. We can distinguish stimuli of different sorts: to
the eye, the ear, the nose, the palate, etc. We can also distinguish
elements of different sorts in the reaction: visual elements, auditory
elements, etc. The latter are defined, not by the stimulus, but by
their intrinsic quality. A visual sensation and a visual image have a
common quality which neither shares with an auditory sensation or
an auditory image. We may then say: a visual image is an
occurrence having the visual quality but not due to a stimulus to the
eye, i.e. not having as a direct causal antecedent the incidence of
light-waves upon the retina. Similarly an auditory image will be an
occurrence having the auditory quality but not due to sound-waves
reaching the ear, and so on for the other senses. This means a
complete abandonment of the attempt to distinguish psychologically
between sensations and images; the distinction becomes solely one
as to physical antecedents. It is true that we can and do arrive at

the distinction without scientific physics, because we find that
certain elements in our integral reactions have the correlations that
make us regard them as corresponding to something external while
others do not—correlations both with the experience of others and
with our own past and future experiences. But when we refine upon
this common-sense distinction and try to make it precise, it becomes
the distinction in terms of physics as stated just now.
We might therefore conclude that an image is an occurrence
having the quality associated with stimulation by some sense-organ,
but not due to such stimulation. In human beings, images seem to
depend upon past experience, but perhaps in more instinctive
animals they are partly due to innate cerebral mechanisms. In any
case dependence upon experience is not the mark by which they are
to be defined. This shows how intimate is the dependence of
traditional psychology upon physics, and how difficult it is to make
psychology into an autonomous science.
There is, however, still a further refinement necessary. Whatever
is included under our present definition is an image, but some things
not included are also images. The sight of an object may bring with
it a visual image of some other object frequently associated with it.
This latter is called an image, not a sensation, because, though also
visual, it is not appropriate to the stimulus in a certain sense: it
would not appear in a photograph of the scene, or in a photograph
of the retina. Thus we are forced to say: the sensation element in
the reaction to a stimulus is that part which enables you to draw
inferences as to the nature of the extra-cerebral event (if any) which
was the stimulus;
9
the rest is images. Fortunately, images and
sensations usually differ in intrinsic quality; this makes it possible to
get an approximate idea of the external world by using the usual
intrinsic differences, and to correct it afterwards by means of the
strict causal definition. But evidently the matter is difficult and
complicated, depending upon physics and physiology, not upon pure
psychology. This is the main thing to be realised about images.

9
I.e. the immediate stimulus, not the “physical
object”.
The above discussion has suggested a definition of the word
“image”. We might have called an event an “image” when it is
recognisably of the same kind as a “percept”, but does not have the
stimulus which it would have if it were a percept. But if this
definition is to be made satisfactory, it will be necessary to substitute
a different word in place of “percept”. For example, in the percept of
a visible object it would be usual to include certain associated tactual
elements, but these must, from our point of view, count as images.
It will be better to say, therefore, that an “image” is an occurrence
recognisably visual (or auditory, etc., as the case may be), but not
caused by a stimulus which is of the nature of light (or sound etc.,
as the case may be), or at any rate only indirectly so caused as a
result of association. With this definition, I do not myself feel any
doubt as to the existence of images. It is clear that they constitute
most of the material of dreams and day-dreams, that they are
utilised by composers in making music, that we employ them when
we get out of a familiar room in the dark (though here the rats in
mazes make a different explanation possible), and that they account
for the shock of surprise we have when we take salt thinking it is
sugar or (as happened to me recently) vinegar thinking it is coffee.
The question of the causation of images—i.e. whether it is in the
brain or in other parts of the body—is not one which it is necessary
to our purposes to decide, which is fortunate, since, so far as I
know, there is not at present any adequate evidence on the point.
But the existence of images and their resemblance to perception is
important, as we shall see in the next chapter.
Images come in various ways, and play various parts. There are
those that come as accretions to a case of sensation, which are not
recognised as images except by the psychologist; these form, for
example, the tactual quality of things we only see, and the visual

quality of things we only touch. I think dreams belong, in part, to
this class of images: some dreams result from misinterpreting some
ordinary stimulus, and in these cases the images are those
suggested by a sensation, but suggested more uncritically than if we
were awake. Then there are images which are not attached to a
present reality, but to one which we locate in the past; these are
present in memory, not necessarily always, but sometimes. Then
there are images not attached to reality at all so far as our feeling
about them goes: images which merely float into our heads in
reverie or in passionate desire. And finally there are images which
are called up voluntarily, for example, in considering how to decorate
a room. This last kind has its importance, but I shall say nothing
more about it at present, since we cannot profitably discuss it until
we have decided what we are to mean by the word “voluntary”. The
first kind, which comes as an accretion to sensation, and gives to
our feeling of objects a certain rotundity and full-bloodedness which
the stimulus alone would hardly warrant, has been considered
already. Therefore what remains for the present is the use of images
in memory and imagination; and of these two I shall begin with
memory.

CHAPTER XVIII
IMAGINATION AND MEMORY
In this chapter we have to consider the two topics of imagination
and memory. The latter has already been considered in Chapter VI,
but there we viewed it from outside. We want now to ask ourselves
whether there is anything further to be known about it by taking
account of what is only perceptible to the person remembering.
As regards the part played by images, I do not think this is
essential. Sometimes there are memory-images, sometimes not;
sometimes when images come in connection with memory, we may
nevertheless know that the images are incorrect, showing that we
have also some other and more reliable source of memory. Memory
may depend upon images, as in the case mentioned above, of the
house where I lived as a child. But it may also be purely verbal. I am
a poor visualiser, except for things I saw before I was ten years old;
when now I meet a man and wish to remember his appearance, I
find that the only way is to describe him in words while I am seeing
him, and then remember the words. I say to myself: “This man has
blue eyes and a brown beard and a small nose; he is short, with a
rounded back and sloping shoulders”. I can remember these words
for months, and recognise the man by means of them, unless two
men having these characteristics are present at once. In this
respect, a visualiser would have the advantage of me. Nevertheless,
if I had made my verbal inventory sufficiently extensive and precise,
it would have been pretty sure to answer its purpose. I do not think
there is anything in memory that absolutely demands images as
opposed to words. Whether the words we use in “thought” are
themselves sometimes images of words, or are always incipient

movements (as Watson contends), is a further question, as to which
I offer no opinion, since it ought to be capable of being decided
experimentally.
The most important point about memory is one which has
nothing to do with images, and is not mentioned in Watson’s brief
discussion. I mean the reference to the past. This reference to the
past is not involved in mere habit memory, e.g. in skating or in
repeating a poem formerly learned. But it is involved in recollection
of a past incident. We do not, in this case, merely repeat what we
did before: then, we felt the incident as present, but now we feel it
as past. This is shown in the use of the past tense. We say to
ourselves at the time “I am having a good dinner”, but next day we
say “I did have a good dinner”. Thus we do not, like a rat in a maze,
repeat our previous performance: we alter the verbal formula. Why
do we do so? What constitutes this reference of a recollection to the
past?
10
10
On this subject, cf. Broad, The Mind and Its
Place in Nature, p. 264 ff., in his chapter on
“Memory”.
Let us take up the question first from the point of view of
sensitivity. The stimulus to a recollection is, no doubt, always
something in the present, but our reaction (or part of it) is more
intimately related to a certain past event than to the present
stimulus. This, in itself, can be paralleled in inanimate objects, for
example, in a gramophone record. It is not the likeness of our
reaction to that called forth on a former occasion that concerns us at
the moment; it is its un-likeness, in the fact that now we have the
feeling of pastness, which we did not have originally. You cannot
sing into a dictaphone “I love you”, and have it say five days hence
“I loved you last Wednesday”; yet that is what we do when we

remember. I think, however, that this feature of memory is probably
connected with a feature of reactions due to association when the
association is cerebral: I think also that this is connected with the
difference in quality that exists usually, though not always, between
images and sensations. It would seem that, in such cases, the
reaction aroused through association is usually different from that
which would have been aroused directly, in certain definite ways. It
is fainter, and has, when attended to, the sort of quality that makes
us call it “imaginary”. In a certain class of cases, we come to know
that we can make it “real” if we choose; this applies, e.g. to the
tactual images produced by visible objects that we can touch. In
such cases, the image is attached by us to the object, and its
“imaginary” character fails to be noticed. These are the cases in
which the association is not due to some accident of our experience,
but to a collocation which exists in nature. In other cases, however,
we are perfectly aware, if we reflect, that the association depends
upon some circumstance in our private lives. We may, for instance,
have had a very interesting conversation at a certain spot, and
always think of this conversation when we find ourselves in this
place. But we know that the conversation does not actually take
place again when we go back to where it happened. In such a case,
we notice the intrinsic difference between the event as a sensible
fact in the present and the event as merely revived by association. I
think this difference has to do with our feeling of pastness. The
difference which we can directly observe is not, of course, between
our present recollection and the past conversation, but between our
present recollection and present sensible facts. This difference,
combined with the inconsistency of our recollection with present
facts if our recollection were placed in the present, is perhaps a
cause of our referring memories to the past. But I offer this
suggestion with hesitation; and, as we shall find when we have
examined imagination, it cannot be the whole truth, though it may
be part of it.
There are some facts that tend to support the above view. In
dreams, when our critical faculty is in abeyance, we may live past

events over again under the impression that they are actually
happening; the reference of recollections to the past must,
therefore, be a matter involving a somewhat advanced type of
mental activity. Conversely, we sometimes have the impression that
what is happening now really happened in the past; this is a well-
known and much discussed illusion. It happens especially when we
are profoundly absorbed in some inward struggle or emotion, so that
outer events only penetrate faintly. I suggest that, in these
circumstances, the quality of sensations approximates to that of
images, and that this is the source of the illusion.
If this suggestion is right, the feeling of pastness is really
complex. Something is suggested by association, but is recognisably
different from a present sensible occurrence. We therefore do not
suppose that this something is happening now; and we may be
confirmed in this by the fact that it is inconsistent with something
that is happening now. We may then either refer the something to
the past, in which case we have a recollection, though not
necessarily a correct one; or we may regard the something as purely
imaginary, in which case we have what we regard as pure
imagination. It remains to inquire why we do sometimes the one and
sometimes the other, which brings us to the discussion of
imagination. I think we shall find that memory is more fundamental
than imagination, and that the latter consists merely of memories of
different dates assembled together. But to support this theory will
demand first an analysis of imagination and then, in the light of this
analysis, an attempt to give further precision to our theory of
memory.
Imagination is not, as the word might suggest, essentially
connected with images. No doubt images are often, even usually,
present when we imagine, but they need not be. A man can
improvise on the piano without first having images of the music he is
going to make; a poet might write down a poem without first
making it up in his head. In talking, words suggest other words, and
a man with sufficient verbal associations may be successfully carried
along by them for a considerable time. The art of talking without

thinking is particularly necessary to public speakers, who must go on
when once they are on their feet, and gradually acquire the habit of
behaving in private as they do before an audience. Yet the
statements they make must be admitted to be often imaginative.
The essence of imagination, therefore, does not lie in images.
The essence of imagination, I should say, is the absence of belief
together with a novel combination of known elements. In memory,
when it is correct, the combination of elements is not novel; and
whether correct or not, there is belief. I say that in imagination there
is “a novel combination of known elements”, because, if nothing is
novel, we have a case of memory, while if the elements, or any of
them, are novel, we have a case of perception. This last I say
because I accept Hume’s principle that there is no “idea” without an
antecedent “impression”. I do not mean that this is to be applied in a
blind and pedantic manner, where abstract ideas are concerned. I
should not maintain that no one can have an idea of liberty until he
has seen the Statue of Liberty. The principle applies rather to the
realm of images. I certainly do not think that, in an image, there can
be any element which does not resemble some element in a
previous perception, in the distinctive manner of images.
Hume made himself an unnecessary difficulty in regard to the
theory that images “copy” impressions. He asked the question:
Suppose a man has seen all the different shades of colour that go to
make up the spectrum, except just one shade. To put the thing in
modern language, suppose he has never seen light of a certain small
range of wave-lengths, but has seen light of all other wave-lengths.
Will he be able to form an image of the shade he has never seen?
Hume thinks he will, although this contradicts the principle. I should
say that images are always more or less vague copies of
impressions, so that an image might be regarded as a copy of any
one of a number of different impressions of slightly different shades.
In order to get a test case for Hume’s question, we should have to
suppose that there was a broad band of the spectrum that the man
had never seen—say the whole of the yellow. He would then, one
may suppose, be able to form images which, owing to vagueness,

might be applicable to orange-yellow, and others applicable to
green-yellow, but none applicable to a yellow midway between
orange and green. This is an example of an unreal puzzle
manufactured by forgetting vagueness. It is analogous to the
following profound problem: A man formerly hairy is now bald; he
lost his hairs one by one; therefore there must have been just one
hair that made the difference, so that while he had it he was not
bald but when he lost it he was. Of course “baldness” is a vague
conception; and so is “copying”, when we are speaking of the way in
which images copy prototypes.
What causes us, in imagination, to put elements together in a
new way? Let us think first of concrete instances. You read that a
ship has gone down on a route by which you have lately travelled;
very little imagination is needed to generate the thought “I might
have gone down”. What happens here is obvious: the route is
associated both with yourself and with shipwreck, and you merely
eliminate the middle term. Literary ability is largely an extension of
the practice of which the above is a very humble example. Take, say:
And all our yesterdays have lighted fools
The way to dusty death. Out, out, brief candle!
Life’s but a walking shadow, a poor player
Who struts and frets his hour upon the stage,
And then is heard no more. It is a tale
Told by an idiot, full of sound and fury
Signifying nothing.
I do not pretend to explain all the associations which led
Shakespeare to think of these lines, but some few are obvious.
“Dusty death” is suggested by Genesis iii. 19: “Dust thou art, and
unto dust shalt thou return”. Having spoken of “lighting fools the
way”, it is natural to think of a “candle”, and thence of a “walking
shadow” being lighted by the candle along the way. From shadows
to players was a well-established association in Shakespeare’s mind;

thus in Midsummer Night’s Dream he says of players: “The best in
this kind are but shadows, and the worst are no worse, if
imagination amend them”. From a “poor player” to a “tale told by an
idiot” is no very difficult transition for a theatre-manager; and
“sound and fury” no doubt often formed part of the tales to which
he had to listen in spite of their “signifying nothing”. If we knew
more about Shakespeare, we could explain more of him in this sort
of way.
Thus exceptional imaginative gifts appear to depend mainly
upon associations that are unusual and have an emotional value
owing to the fact that there is a certain uniform emotional tone
about them. Many adjectives are suitable to death: in a mood quite
different from Macbeth’s, it may be called “noble, puissant and
mighty”. A Chancellor of the Exchequer, thinking of the Death Duties,
might feel inclined to speak of “lucrative death”; nevertheless he
would not, like Vaughan, speak of “dear, beauteous death”.
Shakespeare also would not have spoken of death in such terms, for
his view of it was pagan; he speaks of “that churl death”. So a man’s
verbal associations may afford a key to his emotional reactions, for
often what connects two words in his mind is the fact that they
rouse similar emotions.
The absence of belief that accompanies imagination is a
somewhat sophisticated product; it fails in sleep and in strong and
emotional excitement. Children invent terrors for fun, and then begin
to believe in them. The state of entertaining an idea without
believing it is one involving some tension, which demands a certain
level of intellectual development. It may be assumed that
imagination, at first, always involved belief, as it still does in dreams.
I am not concerned at the moment to define “belief”, but a criterion
is influence on action. If I say “suppose there were a tiger outside
your front door”, you will remain calm; but if I say, with such a
manner as to command belief, “there is a tiger outside your front
door”, you will stay at home, even if it involves missing your train to
the office. This illustrates what I mean when I say that imagination,
in its developed form, involves absence of belief. But this is not true

of its primitive forms. And even a civilised adult, passing through a
churchyard on a dark night, may feel fear if his imagination turns in
the direction of ghosts.
When imagination passes into belief, it does not, as a rule,
become a belief about the past. Generally we place the imagined
object in the present, but not where it would be perceptible to our
senses. If we place it in the past, it is because the past has some
great emotional significance for us. If a person we love has been in
great danger, and we do not know whether he has come through
safely, imagination of his death may lead us to believe that he has
been killed. And often imagination leads us to believe that
something is going to happen. What is common to all such cases is
the emotional interest: this first causes us to imagine an event, and
then leads us to think that it has happened, is happening, or will
happen, according to the circumstances. Hope and fear have this
effect equally; wish-fulfilment and dread-fulfilment are equally
sources of dreams and day-dreams. A great many beliefs have a
source of this kind. But, in spite of psycho-analysis, there are a great
many that have a more rational foundation. I believe that Columbus
first crossed the ocean in 1492, though 1491 or 1493 would have
suited me just as well. I cannot discover that there is any emotional
element in this belief, or in the belief that Semipalatinsk is in Central
Asia. The view that all our beliefs are irrational is perhaps somewhat
overdone nowadays, though it is far more nearly true than the views
that it has displaced.
We must now return to the subject of memory. Memory proper
does not, like imagination, involve a re-arrangement of elements
derived from past experience; on the contrary, it should restore such
elements in the pattern in which they occurred. This is the vital
difference between memory and imagination; belief, even belief
involving reference to the past, may, as we have seen, be present in
what is really imagination though it may not seem to be so to the
person concerned. That being so, we still have to consider what
constitutes the reference to the past, since the view tentatively

suggested before we had considered imagination turns out to be
inadequate.
There is one possible view, suggested, though not definitely
adopted, by Dr. Broad in his chapter on “Memory” already referred
to. According to this view, we have to start from temporal succession
as perceived within what is called the “specious present”, i.e. a short
period of time such that the events that occur throughout it can be
perceived together. (I shall return to this subject presently.) For
example, you can see a quick movement as a whole; you are not
merely aware that the object was first in one place and then in
another. You can see the movement of the second-hand of a watch,
but not of the hour-hand or minute-hand. When you see a
movement in this sense, you are aware that one part of it is earlier
than another. Thus you acquire the idea “earlier”, and you can mean
by “past” “earlier than this”, where “this” is what is actually
happening. This is a logically possible theory, but it seems
nevertheless somewhat difficult to believe. I do not know, however,
of any easier theory, and I shall therefore adopt it provisionally while
waiting for something better.
For the understanding of memory, it is a help to consider the
links connecting its most developed forms with other occurrences of
a less complex kind. True recollection comes at the end of a series of
stages. I shall distinguish five stages on the way, so that recollection
becomes the sixth in gradual progress. The stages are as follows:
1. Images.—As we have seen, images, at any rate in their
simpler parts, in fact copy past sensations more or less vaguely,
even when they are not known to do so. Images are “mnemic”
phenomena, in the sense that they are called up by stimuli formerly
associated with their prototypes, so that their occurrence is a result
of past experience according to the law of association. But obviously
an image which in fact copies a past occurrence does not constitute
a recollection unless it is felt to be a copy.
2. Familiarity.—Images and perceptions may come to us, and so
may words or other bodily movements, with more or less of the

feeling we call “familiarity”. When you recall a tune that you have
heard before, either by images or by actually singing it, part of what
comes to you may feel familiar, part unfamiliar. This may lead you to
judge that you have remembered the familiar part rightly and the
unfamiliar part wrongly, but this judgment belongs to a later stage.
3. Habit-Memory.—We have already discussed this in Chapter VI.
People say they remember a poem if they can repeat it correctly. But
this does not necessarily involve any recollection of a past
occurrence; you may have quite forgotten when and where you read
the poem. This sort of memory is mere habit, and is essentially like
knowing how to walk although you cannot remember learning to
walk. This does not deserve to be called memory in the strict sense.
4. Recognition.—This has two forms. (a) When you see a dog,
you can say to yourself “there is a dog”, without recalling any case in
which you have seen a dog before, and even without reflecting that
there have been such cases. This involves no knowledge about the
past; essentially it is only an associative habit. (b) You may know “I
saw this before”, though you do not know when or where, and
cannot recollect the previous occurrence in any way. In such a case
there is knowledge about the past, but it is very slight. When you
judge: “I saw this before”, the word “this” must be used vaguely,
because you did not see exactly what you see now, but only
something very like this. Thus all that you are really knowing is that,
on some past occasion, you saw something very like what you are
seeing now. This is about the minimum of knowledge about the past
that actually occurs.
5. Immediate Memory.—I come now to a region intermediate
between sensation and true memory, the region of what is
sometimes called “immediate memory”. When a sense-organ is
stimulated, it does not, on the cessation of the stimulus, return at
once to its unstimulated condition: it goes on (so to speak) vibrating,
like a piano-string, for a short time. For example, when you see a
flash of lightning, your sensation, brief as it is, lasts much longer
than the lightning as a physical occurrence. There is a period during

which a sensation is fading: it is then called an “acoleuthic”
sensation. It is owing to this fact that you can see a movement as a
whole. As observed before, you cannot see the minute-hand of a
watch moving, but you can see the second-hand moving. That is
because it is in several appreciably different places within the short
time that is required for one visual sensation to fade, so that you do
actually, at one moment, see it in several places. The fading
sensations, however, feel different from those that are fresh, and
thus the various positions which are all sensibly present are placed
in a series by the degree of fading, and you acquire the perception
of movement as a process. Exactly the same considerations apply to
hearing a spoken sentence.
Thus not only an instant, but a short finite time is sensibly
present to you at any moment. This short finite time is called the
“specious present”. By the felt degree of fading, you can distinguish
earlier and later in the specious present, and thus experience
temporal succession without the need of true memory. If you see me
quickly move my arm from left to right, you have an experience
which is quite different from what you would have if you now saw it
at the right and remembered that a little while ago you saw it at the
left. The difference is that, in the quick movement, the whole falls
within the specious present, so that the entire process is sensible.
The knowledge of something as in the immediate past, though still
sensible, is called “immediate memory”. It has great importance in
connection with our apprehension of temporal processes, but cannot
count as a form of true memory.
6. True Recollection.—We will suppose, for the sake of
definiteness, that I am remembering what I had for breakfast this
morning. There are two questions which we must ask about this
occurrence: (a) What is happening now when I recollect? (b) What
is the relation of the present happening to the event remembered?
As to what is happening now, my recollection may involve either
images or words; in the latter case, the words themselves may be
merely imagined. I will take the case in which there are images

without words, which must be the more primitive, since we cannot
suppose that memory would be impossible without words.
The first point is one which seems so obvious that I should be
ashamed to mention it, but for the fact that many distinguished
philosophers think otherwise. The point is this: whatever may be
happening now, the event remembered is not happening. Memory is
often spoken of as if it involved the actual persistence of the past
which is remembered; Bergson, e.g. speaks of the interpenetration
of the present by the past. This is mere mythology; the event which
occurs when I remember is quite different from the event
remembered. People who are starving can remember their last meal,
but the recollection does not appease their hunger. There is no
mystic survival of the past when we remember; merely a new event
having a certain relation to the old one. What this relation is, we
shall consider presently.
It is quite clear that images are not enough to constitute
recollection, even when they are accurate copies of a past
occurrence. One may, in a dream, live over again a past experience;
while one is dreaming, one does not seem to be recalling a previous
occurrence, but living through a fresh experience. We cannot be said
to be remembering, in the strict sense, unless we have a belief
referring to the past. Images which, like those in dreams, feel as if
they were sensations, do not constitute recollection. There must be
some feeling which makes us refer the images to a past prototype.
Perhaps familiarity is enough to cause us to do so. And perhaps this
also explains the experience of trying to remember something and
feeling that we are not remembering it right. Parts of a complex
image may feel more familiar than other parts, and we then feel
more confidence in the correctness of the familiar parts than in that
of the others. The conviction that the image we are forming of a
past event is wrong might seem to imply that we must be knowing
the past otherwise than by images, but I do not think this conclusion
is really warranted, since degrees of familiarity in images suffice to
explain this experience.

(b) What is the relation of the present happening to the event
remembered? If we recollect correctly, the several images will have
that kind of resemblance of quality which images can have to their
prototypes, and their structure and relations will be identical with
those of their prototypes. Suppose, for instance, you want to
remember whether, in a certain room, the window is to the right or
left of the door as viewed from the fireplace. You can observe your
image of the room, consisting (inter alia) of an image of the door
and an image of the window standing (if your recollection is correct)
in the same relation as when you are actually seeing the room.
Memory will consist in attaching to this complex image the sort of
belief that refers to the past; and the correctness of memory
consists of similarity of quality and identity of structure between the
complex image and a previous perception.
As for the trustworthiness of memory, there are two things to be
said. Taken as a whole, memory is one of the independent sources
of our knowledge; that is to say, there is no way of arriving at the
things we know through memory by any argument wholly derived
from things known otherwise. But no single memory is obliged to
stand alone, because it fits, or does not fit, into a system of
knowledge about the past based upon the sum-total of memories.
When what is remembered is a perception by one or more of the
public senses, other people may corroborate it. Even when it is
private, it may be confirmed by other evidence. You may remember
that you had a toothache yesterday, and that you saw the dentist to-
day; the latter fact may be confirmed by an entry in your diary. All
these make a consistent whole, and each increases the likelihood of
the other. Thus we can test the truth of any particular recollection,
though not of memory as a whole. To say that we cannot test the
truth of memory as a whole is not to give a reason for doubting it,
but merely to say that it is an independent source of knowledge, not
wholly replaceable by other sources. We know that our memory is
fallible, but we have no reason to distrust it on the whole after
sufficient care in verification has been taken.

The causation of particular acts of recollection seems to be
wholly associative. Something in the present is very like something
in the past, and calls up the context of the past occurrence in the
shape of images or words; when attention falls upon this context,
we believe that it occurred in the past, not as mere images, and we
then have an act of recollection.
There are many difficult problems connected with memory which
I have not discussed, because they have an interest which is more
purely psychological than philosophical. It is memory as a source of
knowledge that specially concerns the philosopher.

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J2EE Technology in Practice Building Business Applications with the Java 2 Platform 1st Edition Rick Cattell

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