KNOWLEDGE REPRESENTATION ISSUES.ppt

Knowledge Representation Issues

Introduction
• For the purpose of solving complex problems encountered in
AI, we need both a large amount of knowledge and some
mechanism for manipulating that knowledge to create
solutions to new problems.
• A variety of ways of representing knowledge (facts) have been
exploited in AI programs. In all variety of knowledge
representations , we deal with two kinds of entities.
A. Facts: Truths in some relevant world. These are the things we
want to represent.
B. Representations of facts in some chosen formalism . these are
things we will actually be able to manipulate.

• One way to think of structuring these entities is at two
levels :
(a) the knowledge level, at which facts are described, and
(b) the symbol level, at which representations of objects
at the knowledge level are defined in terms of
symbols that can be manipulated by programs.
• The facts and representations are linked with two-way
mappings.
• This link is called representation mappings.
• The forward representation mapping maps from facts
to representations.
• The backward representation mapping goes the other
way, from representations to facts.

• One common representation is natural language (particularly
English) sentences.
• Regardless of the representation for facts we use in a program
,we may also need to be concerned with an English
representation of those facts in order to facilitate getting
information into and out of the system.
• We need mapping functions from English sentences to the
representation we actually use and from it back to sentences.

Representations and Mappings
• In order to solve complex problems encountered in artificial
intelligence, one needs both a large amount of knowledge
and some mechanism for manipulating that knowledge to
create solutions.
 Knowledge and Representation are two distinct entities.
They play central but distinguishable roles in the intelligent
system.
Knowledge is a description of the world. It determines a
system’s competence by what it knows.
Moreover, Representation is the way knowledge is
encoded. It defines a system’s performance in doing
something.
Different types of knowledge require different kinds of
representation.

KNOWLEDGE REPRESENTATION ISSUES.ppt

Let's look at a simple example using
mathematical logic as the representational
formalism. Consider the English sentence:
Spot is a dog.
• The fact represented by that English sentence
can also be represented in logic as:
dog(Spot)
• Suppose that we also have a logical
representation of the fact that all dogs
havetails:
• For all x: dog(x) -> hastail{x}

• Then, using the deductive mechanismsof
logic, we may generate the new
representation object:
hastail(Spot)
• Using an appropriate backward mapping
function, we could then generate the English
sentence:
Spot has a fail.

Approaches to knowledge Representation
• Representational adequacy:- the ability to represent all of the
kinds of knowledge that are needed in that domain.
• Inferential Adequacy:- the ability to manipulate the
representation structures in such a way as to derive new
structures corresponding to new knowledge inferred from old.
• Inferential Efficiency:- the ability to incorporate into the
knowledge structure additional information that can be used
to focus the attention of the inference mechanism in the most
promising directions.
• Acquisitional Efficiency:- the ability to acquire new
information easily. The simplest case involves direct insertion
by a person, of new knowledge into the database.

• Simple Relational knowledge
The simplest way to represent declarative facts is
as a set of relations of the same sort used in database
system.
The reason that this representation is simple is that
standing alone it provides very weak inferential
capabilities But knowledge represented in this form
may serve as the input to more powerful inference
engines.

To answer question “Whois the heaviest
player?”
But if a procedure for finding the heaviest
player is provided, then these facts will enable
the procedure to compute an answer.
If, instead, we are provided with a set of rules
for deciding which hitter to put up against a
given pitcher (based on right- and left-
handedness, say), then this same relation can
provide at least some of the information
required by those rules.

• Inheritable Knowledge
Structure must be designed to corresponding to
the inference mechanism that are desired.
One of the most useful forms of inference is
Property inheritance
– elements inherit values from being members of a
class.
– data must be organised into a hierarchy of classes.

Property inheritance algorithm
• The algorithm to retrieve a value for an attribute of
an instance object:
1.Find the object in the knowledge base
2.If there is a value for the attribute report it
3.Otherwise look for a value of instance. if none, fail
4.Otherwise go to that node and find a value for the
attribute and then report it

5.Otherwise search through using isa until a value is
found for the attribute.
a)get the value of the isa attribute and move to that
node
b)see if there is a value for the attribute ,if there is ,
report it.

Baseball-Player
isa: Adult-Male
bats: (EQUAL handed}
height: 6-1
batting-average: 252
Fig. 4.6 Viewing a Node as a Frame

team(Pee-Wee-Reese) = Brooklyn-Dodgers.
batting-average(Three-Finger Brown) = .106.
height(Pee-Wee-Reese) = 6-1.(default
inheritance)
bats(Three-Finger-Brown) = Right.

• Inferential Knowledge
Represents knowledge as formal logic. Based on
reasoning from facts or from other inferential
knowledge. Useless unless there is also as inference
procedure that can exploit it.
• Procedural Knowledge (Imperative Knowledge or
operational knowledge)
-specifies what to do when.
-Procedural knowledge can be represented in
programs in many ways. The most common way is
simply as code (in some programming language such as
LISP) for doing something.

-The machine uses the knowledge when it
executes the code to perform a task.
-The most commonly used technique for
representing procedural knowledge in AI
programs is the use of production rules.(if –
then)

Issues in Knowledge Representation
• Are any attributes of objects so basic that they have
been occurred in almost every problem domain?
• Are there any important relationships that exist
among attributes of objects?
• At what level should knowledge be represented?How
should sets of objects be represented?
• How can relevant part be accessed when they are
needed?

Important Attributes
• There are two attributes that are of very general
significance, instance and isa
• These attributes are important because they support
property inheritance

Relationships among Attributes
• The attributes that we use to describe objects are
themselves entities that we represent.
• What properties do they have independent of the
specific knowledge they enco
• There are four such properties that deserve mention
here:
i) Inverses
ii) Existence in an isa hierarchy
iii)Techniques for reasoning about values
iv)Single-valued attributes

Inverses
• Entities in the world are related to each other in
many different ways.
• Attributes are those relationships.
• The first is to represent both relationships in a single
representation that ignores focus.
• Logical representations are usually interpreted as
doing this. For example, the assertion:
team( Pee-Wee-Reese, Brooklyn-Dodgers)
can equally easily be interpreted as a statement about
Pee Wee Reese or about the Brooklyn Dodgers.

• The second approach is to use attributes that focus
on a single entity but to use them in pairs, one the
inverse of the other.
-one associated with Pee Wee Reese:
team = Brooklyn-Dodgers
-one associated with Brooklyn Dodgers:
team-members = Pee-Wee-Reese....
This is the approach that is taken in semantic net and
frame-based systems.

An Isa Hierarchy ofAttributes
• generalization-specialization relationships are
important for attributes

Techniques for Reasoning about Values
• Sometimes values of attributes are specified
explicitly when a knowledge base is created.
• Several kinds of information can play a role in this
reasoning, including:
- Information about the type of the value. For
example, the value of Height must be a number
measured in a unit of length.
-Constraints on the value, often stated in terms
of related entities. For example, the age of a person
cannot be greater than the age of either of that
person’s parents.

-Rules for computing the value whenit is needed.
We showed an example of such a rule in Fig. 4.5 for
the bats attribute. These rules are called backward
rules, Such rules have also been called if-needed
rules,
-Rules that describe actions that should be taken
if a value ever becomes known. These rules are
called forward rules, or sometimes if-added rules.

Single-Valued Attributes
• A specific but very useful kind of attribute is one that
is guaranteed to take a unique value. For example, a
baseball player can, at any one time, have only a
single height and be a member of only one team.
• If there is already a value present for one of these
attributes and a different value is asserted, then one
of two things has happened.
• Either a change has occurred in the world or there is
now a contradiction in the knowledge base that
needs to be resolved.

• Knowledge-representation systems have taken
several different approaches to providing support for
single-valued attributes, including:
-Introduce an explicit notation for temporal
interval. If two different values are ever asserted for
the same temporal interval, signal a contradiction
automatically.
-Assume that the only temporal interval that is of
interest is now. So if a new value is asserted, replace
the old value.
-

-Provide no explicit support. Logic-based systems
are in this category. But in these systems,
knowledgebase builders can add axioms that state
that if an attribute has one value then it is known not
to have all other values.

Choosing the Granularity of Representation
Regardless of the particular representation formalism
we choose, it is necessary to answer the question
“At what level of detail should the world be
represented?” Another way this question is often
phrased is “What should be our primitives?”
Should there be a small number of low-level ones or
should there be a target number covering a range of
granularities?

• Suppose we are interested in the following fact:
John spotted Sue.
• We could represent this as!
spotted(agent(John),
object(Sue})
• Such a representation would makeit easy to answer
questions suchas:
Who spotted Sue?
• But now suppose we want to know:
Did John see Sue?

• The obvious answer is “yes,” but given only the one
fact we have, we cannot discover that answer.
• We could, of course, add other facts, such as
spotted(x, y) -> saw(x, y)
• We could then infer the answer to the question.
• An alternative solution to this problem is to
represent the fact that spotting is really a special
type of seeing explicitly in the representation of the
fact. We might write something such as
saw(agent(John),object(Sue),timespan(briefly)})

• In this representation, we have broken the idea of
spotting apart into more primitive concepts of seeing
and timespan.
• Using this representation, the fact that John saw Sue
is immediately accessible. But the fact that he
spotted her is more difficult to get to.

• The major advantage of converting all statements
into a representation in terms of a small set of
primitives is that the rules that are used to derive
inferences from that knowledge need be written only
in terms of the primitives rather than in terms of the
many ways in which the knowledge may originally
have appeared.

• There are several arguments against the use of low-
level primitives.
i)simple high-level facts may require a lot of
storage when broken down into primitives. Much of
that storage is really wasted since the low-level
rendition of a particular high- level concept will
appear many times, once for each time the high-level
concept is referenced.

ii) if knowledge is initially presented to the
system in a relatively high-level form, such as English,
then substantial work must be done to reduce the
knowledge into primitive form.
iii) In many domains, it is not at all clear what the
primitives should be. And even in domains in which
there may be an obvious set of primitives, there may
not be enough information present in each use of
the high-level constructs to enable them to be
converted into their primitive components.

• choosing the correct granularity of representation for
a particular body of knowledge is not easy.
• Clearly, the lower the level we choose, the less
inference required to reason with it in some cases,
but the more conference required to create the
representation from English and the more room it
takes to store, since many inferences will be
represented many times.
• The answer for any particular task domain must
come to a large extent from the domain itself —to
what use is the knowledge to be put?

Representing Sets of Objects
• It is important to be able to represent sets of objects
for several reasons.
• One is that there are some properties that are true
of sets that are not true of the individual members of
a set. As examples, consider the assertions that are
being made in the sentences “There are more sheep
than people in Australia” and “English speakers
can be found all over the world.”

• The only way to represent the facts described in
these sentences is to attach assertions to the sets
representing people, sheep, and English speakers,
since, for example, no single English speaker can be
found al! over the world.
• The other reason that it is important to be able to
represent sets of objects is that if a property is true
ofall (or even most) elements of a set, then it is more
efficient to associate it once with the set rather than
to associate it explicitly with every element of the
set.

• There are three obvious ways in which sets may be
represented.
• The simplest is just by a name.
• There are two ways to state a definition of a set and
its elements.
• The first is to list the members. Such a specification is
called an extensional definition.
• The second is to provide a rule that, when a
particular object is evaluated, returns true or false
depending on whether the object is in the set or not.
Such a rule is called an intensional definition.

Ex:
• an extensional description of the set of our
sun’s planets on which people live is {Earth}.
• An intensional description is
{x : sun-planet(x) / human-inhabited(x)}

• Intensional representations have two important
properties that extensional ones lack,
• however. The first is that they can be used to
describe infinite sets and sets not all of whose
elements are explicitly known.
• Thus we can describe intensionally such sets as
prime numbers (of which there are infinitely many)
or kings of England (even though we do not know
who all of them are or even how many of them there
have been).

• The second thing we can do with intensional
descriptions is to allow them to depend on
parameters that can change, such as time or spatial
location. If we do that, then the actual set that is
represented by the description will change as a
function of the value of those parameters.

• To see the effect of this, consider the sentence, “The
president of the United States used to be a Democrat,”
uttered when the currentpresident is a Republican.
• This sentence can mean two things.
-The first is that the specific person who is now president was
once a Democrat. This meaning can be captured
straightforwardly with an extensional representation of “the
president of the United States.”
We just specify the individual, But there is a second meaning,
namely that there was once someone who was the president
and who was a Democrat.

• To represent the meaning of ‘‘the president of the
United States” given this interpretation requires
an intensional description that depends on time.
• Thus we might write president(t), where
president is some function that maps instances of
time onto instances of people, namely U.S.
presidents.

Finding the Right Structures as Needed
• For example, suppose we have a script (a description
of a class of events in terms of contexts, participants,
and subevents) that describes the typical sequence
of events in a restaurant. This script would enable us
to take a text such as
John went to Steak and Ale last night. He ordered a
large rare steak, paid his bill, and left.
and answer ‘‘yes” to the question
Did John eat dinner last night?

• Notice that nowhere in the story was John’s eating
anything mentioned explicitly. But the fact that when
one goes to a restaurant one eats will be contained
in the restaurant script.
• If we know in advance to use the restaurant script,
then we can answer the question easily.
• But in order to be able to reason about a variety of
things, a system must have many scripts for
everything from going to work to sailing around the
world.
• How will it select the appropriate one each time? For
example, nowhere in our story was the word
“restaurant” mentioned.

• In fact, in order to have access to the right structure
for describing a particular situation, it is necessary to
solve all of the following problems.*
a)How to perform an initial selection of the most
appropriate structure,
b)How to fill in appropriate details from the current
situation.
c) How to find a better structure if the one chosen
initially turns out not to be appropriate.

d) What to do if none of the available
structures is appropriate.
e)When to create and remember a new
structure.

Selecting an initial structure
• Selecting candidate knowledge structures to match a
particular problem-solving situation is a hard
problem:
• There are several ways in which it can be done. Three
important approaches are the following:
-Index the structures directly by the significant
English words that can be used to describe them.
For example, let each verb have associated
with it a structure that describes its meaning. This is
the approach taken in conceptual dependency theory.

For example, the word “fly” has a different meaning in
each of the following sentences:
John flew to New York. (He rode in a plane from
one place to another.)
John flew a kite. (He held a kite that was up inthe
air.)
John flew down the street. (He moved very
rapidly.)
John flew into a rage. (An idiom)
• Another problem with this approach is that it is only
useful when there is an English description of the
problem to be solved.

-Consider each major concept as a pointer
to all of the structures (such as scripts} in which
it might be involved, This may produce several
sets of prospective structures.
For example, the concept Steak might
point to two scripts, one for restaurant and
one for supermarket. The concept Bill might
point to a restaurant and a shopping script.
Take the intersection of those sets to get
the structure(s}, preferably precisely one, that
involves all the content words.

• One important problem with this method is that if
the problem description contains any even slightly
extraneous concepts, then the intersection of their
associated structures will be empty.
Ex: “John rode his bicycle to Steak and Ale last
night.”
• Another problem is that it may require a great deal
of computation to compute all of the possibility sets
and then to intersect them. However, if computing
such sets and intersecting them could be done in
parallel, then the time required to produce an
answer would be reasonable even if the total
number of computations is large.

-Locate one major clue in the problem
description and use it to select an initial structure.
• As other clues appear, use them to refine the initial
selection or to make a completely new one if
necessary. For a discussion of this approach, see
Charniak [1978].
• The major problem with this method is that in some
situations there is not an easily identifiable major
clue.
• A second problem is that it is necessary to anticipate
which clues are going to be important and which are
not. But the relative importance of clues can change
dramatically from one situation to another.

Revising the Choice When Necessary
• Once we find a candidate knowledge
structure, we must attempt to do a detailed
match of it to the problem at hand.
• Depending on the representation we are
using, the details of the matching process will
vary.
• It may require variables to be bound to
objects. It may require attributes to have their
values compared.

• In any case, if values that satisfy the required
restrictions as imposed by the knowledge structure
can be found, they are put into the appropriate
places in the structure. If no appropriate values can
be found, then a new structure must be selected.
• There are a variety of things that can be done:
1.Select the fragments of the current structure that
do correspond to the situation and match them against
candidate alternatives. Choose the best match.
If the current structure was at all close to being
appropriate, much of the work that has been done to
build sub-structures to fit into it will be preserved.

2. Make an excuse for the current structure’s
failure and continue to use it. For example, a proposed
chair with only three legs might simply be broken. Or
there might be another object in front of it which
occludes one leg.
Part of the structure should contain information
about the features for which it is acceptable to make
excuses.
Also, there are general heuristics, such as the fact
that a structure is more likely to be appropriate if a
desired feature is missing (perhaps because it is hidden
from view) than if an inappropriate feature is present.
For example, a person with one leg is more plausible
than a person with a tail.

3.Refer to specific stored links between
structures to suggest new directions in which to
explore.Refer fig.4.11

4. If the knowledge structures are stored in an isa
hierarchy, traverse upward in it until a structure is
found that is sufficiently general that it does net
conflict with the evidence. Either use this structure if it
is specific enough to provide the required knowledge or
consider creating a new structure just below the
matching one.

THE FRAME PROBLEM
• So far in this chapter, we have seen several methods
for representing knowledge that would allow us to
form complex state descriptions for a search
program, Another issue concerns how to represent
efficiently sequences of problem states that arise
from a search process.
• For complex ill-structured problems, this can be a
serious matter.

• Consider the world of a household robot, There are
many objects and relationships in the world, and a
state description must somehow include facts like
on(Plant!2, Table34), under(Table34, Window13),
and in(Table34,Room 15).
• One strategy is to store each state description as a
list of such facts.
• But what happens during the problem-solving
process if each of those descriptions is very long?
• Most of the facts will not change from one state to
another, yet each fact will be represented once at
every node, and we will quickly run out of memory.

• Furthermore, we will spend the majority of our time
creating these nodes and copying these facts—most
of which do not change often—from one node to
another. For example, in the robot world, we could
spend lot of time recording above(Ceiling, Floor) at
every node.
• AII of this is, of course, in addition to the rea!
Problem of figuring out which facts should be
different at each node.

• This whole problem of representing the facts that
change as well as those that do not is known as the
frame problem [McCarthy and Hayes, 1969].
• In some domains, the only hard part is representing
all the facts.
• In others, though, figuring out which ones change is
nontrivial.
• For example, in the robot world, there might be a
table with a plant on it under the window. Suppose
we move the table to the centre of the room.
• We must also infer that the plant is now in the centre
of the room too but that the window is not.

• To support this kind of reasoning, some systems
make use of an explicit set of axioms called frame
axioms, which describe all the things that do not
change when a particular operator is applied in state
n to produce state n + 1.
• Thus, in the robot domain, we might write axioms
such as
color(x,y, s1,;) ˄ move(x, s1, s2) ->color(x,y, s2)
• “If x has color y in state s, and the operation of
moving x is applied in state s, to produce state
s3, then the color of x in 5, is still y.”

• But how do we know what changes in the problem
state description need to be undone?
• For example, what do we have to change to undo the
effect of moving the table to the centre of the room?
• There are two ways this problem can be solved:

-Do not modify the initial state description at all.
• At each node, store an indication of the specific
changes that should be made at this node.
• Whenever it is necessary to refer to the description
of the current problem state, look at the initial state
description and also look back through all the nodes
on the path from the start state to the current state.
This is what we did in our solution to the crypt-
arithmetic problem in Section 3.5.
• This approach makes backtracking very easy, but it
makes referring to the state description fairly
complex.

-Modify the initial state description as
appropriate, but also record at each node an indication
of what to
do to undo the move should it ever be necessary to
backtrack through the node.
Then, whenever it is necessary to backtrack,
check each node along the way and perform the
indicated operations on the state description.

KNOWLEDGE REPRESENTATION ISSUES.ppt

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