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18.2 Time
How are we to think about time?
Real line: extending both into infinite past and infinite future
Integer: countable from beginning with 0 at „big bang‟
[James 1984, Allen 1983]
Time is something that events and processes occur in.
“Interval” : containers for events and processes.
Predicate calculus used for describing interval
Occurs(E, I) : some event or process E, occupies the interval I.
Interval has starting and ending time points.](https://image.slidesharecdn.com/18commonknowledge-150601001711-lva1-app6891/85/18-common-knowledge-7-320.jpg)
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18.3 Knowledge Representation by Networks
18.3.1 Taxonomic Knowledge
The entities of both commonsense and expert domains
can be arranged in hierarchical structures that organize
and simplify reasoning.
CYC system [Guha & Lenat 1990]
Taxonomic hierarchies : encoded either in networks or
data structure called frames.
Example
“Snoopy is a laser printer, all laser printers are printers, all
printers are machines.”
Laser_printer(Snoopy)
(x)[Laser_printer(x) Printer(x)]
(x)[Printer(x) Office_machine(x)]](https://image.slidesharecdn.com/18commonknowledge-150601001711-lva1-app6891/85/18-common-knowledge-9-320.jpg)
18 common knowledge
- 2. 2 Outline The Commonsense World Time Knowledge Representations by Networks Additional Readings and Discussion
- 3. 3 18.1 The Commonsense World What Is Commonsense Knowledge? Most people know the fact that a liquid fall out if the cup is turned upside down. But how can we represent it ? Commonsense knowledge If you drop an object, it will fall. People don‟t exist before they are born. Fish live in water and will die if taken out. People buy bread and milk in a grocery store. People typically sleep at night.
- 4. 4 18.1.2 Difficulties in Representing Commonsense Knowledge How many will be needed by a system capable of general human-level intelligence? No on knows for sure. No well-defined frontiers Knowledge about some topics may not be easily captured by declarative sentences. Description of a human face Many sentences we might use for describing the world are only approximations.
- 5. 5 18.1.3 The Importance of Commonsense Knowledge Machine with commonsense The knowledge such a robot would have to have! Commonsense knowledge for expert systems To recognize outside of the specific area, to predict more accurately. Commonsense for expanding the knowledge of an expert system To understanding natural language
- 6. 6 18.1.4 Research Areas Currently not available system with commonsense but, Object and materials : describing materials and their properties Space : formalizing various notions about space Physical properties : mass, temperature, volume, pressure, etc. Physical Processes and events : modeling by differential equations v.s. qualitative physics without the need for exact calculation Time : developing techniques for describing and reasoning about time
- 7. 7 18.2 Time How are we to think about time? Real line: extending both into infinite past and infinite future Integer: countable from beginning with 0 at „big bang‟ [James 1984, Allen 1983] Time is something that events and processes occur in. “Interval” : containers for events and processes. Predicate calculus used for describing interval Occurs(E, I) : some event or process E, occupies the interval I. Interval has starting and ending time points.
- 8. 8 그림 18.1 Figure 18.1 Relation between Intervals
- 9. 9 18.3 Knowledge Representation by Networks 18.3.1 Taxonomic Knowledge The entities of both commonsense and expert domains can be arranged in hierarchical structures that organize and simplify reasoning. CYC system [Guha & Lenat 1990] Taxonomic hierarchies : encoded either in networks or data structure called frames. Example “Snoopy is a laser printer, all laser printers are printers, all printers are machines.” Laser_printer(Snoopy) (x)[Laser_printer(x) Printer(x)] (x)[Printer(x) Office_machine(x)]
- 10. 10 18.3.2 Semantic Networks Definition : graph structures that encode taxonomic knowledge of objects and their properties Two kinds of nodes Nodes labeled by relation constants corresponding to either taxonomic categories or properties Nodes labeled by object constants corresponding to objects in the domain Three kinds of arcs connecting nodes Subset arcs (isa links) Set membership arcs (instance links) Function arcs
- 11. 11 그림 18.2 Figure 18.2 A Semantic network
- 12. 12 18.3.3 Nonmonotonic Reasoning in Semantic Networks Reasoning in ordinary logic is monotonic. Because adding axioms to a logical system does not diminish the set of theorems that can be proved. We must retract the default inference if new contradictory knowledge arrives. Default inference : barring knowledge to the contrary, we are willing to assume are true. Example of nonmonotonic reasoning : cancellation of inheritance. By default, the energy source of office machines is electric wall outlet. But the energy source of a robot is a battery.
- 13. 13 Figure 18.3 A Semantic Network for Default Reasoning Adding another function arc Contradiction from property inheritance can be resolved by the way in which information about the most specific categories takes precedence over less specific categories.
- 14. 14 18.3.4 Frames Frame is a Data structure which has a name and a set of attribute- value pairs (slots). The frame name corresponds to a node in a semantic network. The attributes (slot names) correspond to the names of arcs associated with this node The values (slot fillers)correspond to nodes at the other ends of these arcs. Semantic networks and frames do have difficulties in expressing certain kinds of knowledge Disjunctions, negations, nontaxonomic knowledge Hybrid system : KRYPTON, CLASSIC Use terminological logic (employing hierarchical structures to represnent entities, classes, and properties and logical expressions for other information).
- 15. 15 Figure 18.5 A Frame