Knowledge Representation using Structured Objects: Semantic Nets and Frames, Slides of Introduction to Computing

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CS613 INTRODUCTION TO

ARTIFICIAL INTELLIGENCE

Lecture 8

Knowledge Representation

Common Knowledge Representations

• Production Rules
• Semantic Nets
• Schemata and Frames
• Formal Logic

Prolog

• Prolog was developed for AI applications.
• It specifies rules as Horn clauses, a subset of predicate logic.
• Example male( albert ). male( edward ). female( alice ). female( victoria ). parents( edward , victoria , albert ). parents( alice , victoria , albert ). sisterof( X , Y ) : - female( X ) , parents( X , M , F ) , parents( Y , M , F ).

Prolog Expert System

% Automotive Diagnostic Expert System

defect_may_be(drained_battery) :- user_says(starter_was_ok, yes), user_says(starter_is_ok, no).

defect_may_be(wrong_gear) :- user_says(starter_was_ok, no).

defect_may_be(fuel_system) :- user_says(starter_was_ok, yes), user_says(fuel_is_ok, no).

Semantic nets

• Originally developed by Quillian as a model for human memory
• knowledge is represented as a collection of concepts, represented by nodes (shown as boxes in the diagram), connected together by relationships, represented by arcs (shown as arrows in the diagram).
• Nodes represent objects, concepts, situations
• Edges represent relationships
• certain arcs - particularly isa arcs - allow inheritance of properties.

Semantic nets

• Developments of the semantic nets idea:
  • psychological research into whether human memory really was organised in this way.
  • used in the knowledge bases in certain expert systems: e.g. PROSPECTOR.
  • special-purpose languages have been written to express knowledge in semantic nets.

Semantic Nets

• Relationships
  • Frequently used: IS-A, A-KIND-OF, PART-OF
  • Can be specified by the designer
• Attributes
  • Can be added to the nodes
• Advantages
  • Easy to encode and understand
• Disadvantages
  • May become large and lead to enormous searches

Semantic Networks

• The ISA (is-a) or AKO (a- kind-of) relation is often used to link instances to classes, classes to superclasses
• Some links (e.g. hasPart) are inherited along ISA paths.
• The semantics of a semantic net can be relatively informal or very formal - often defined at the implementation level

isa

isa

isa isa

Robin

Bird

Animal

Rusty Red

hasPart

Wing

For example, the following:

Inference by Inheritance

• One of the main kinds of reasoning

done in a semantic net is the

inheritance of values along the

subclass and instance links.

• Semantic networks differ in how they

handle the case of inheriting multiple

different values.

• All possible values are inherited, or
• Only the “lowest” value or values are inherited

Conflicting inherited values

Semantic nets

• Problems with semantic nets
  • logical inadequacy - vagueness about what types and tokens really mean.
  • heuristic inadequacy – finding a specific piece of information could be chronically inefficient.
  • trying to establish negation is likely to lead to a combinatorial explosion.
  • "spreading activation" search is very inefficient, because it is not knowledge- guided.

Semantic nets

• Attempted improvements
  • building search heuristics into the

network.

• more sophisticated logical structure,

involving partitioning.

• these improvements meant that the

formalism’s original simplicity was

lost.