Introduction to AI - Week 6

Knowledge
Representation I

Definition of Knowledge

"Knowledge is the symbolic representation of aspects of some named universe of discourse" [Frost, p. 11]
The universe of discourse may be the actual universe or a fictional one, one in the future, or in some belief.
Examples of pieces of knowledge:

Definition of Data

"We define `data' as `the symbolic representation of simple aspects of some named universe of discourse' " [Frost, p. 12]
That is, data is a special case of knowledge.
Examples of pieces of data:

Knowledge Representation in Natural Language

Expressiveness of natural language:

Problems with natural language:

Correct: "When they left the theatre it was pouring, hence they remembered that they forget the umbrella in the cloakroom."
Incorrect: "When they left the cinema the moon was shining, hence they remembered that they forget the umbrella in the cloakroom."


Knowledge Representation in Database Systems

Example:

person record = {    name : max 20 characters
                      age  : 3 digits in range 000-120
                      sex  : male or female
           marital status : married, bachelor, spinster, 
                               divorced, widowed, or engaged
   first names of children : up to 10 names each max 
                               15 characters }

An Instance

A record structure:
-------- ------------- -------------
 J J ADAMS
-------- ------------- -------------
 025
-------- ------------- -------------
 male
-------- ------------- -------------
 married
-------- ------------- -------------
           Sally
-------- ------------- -------------
           Richard
-------- ------------- -------------
           Bob
-------- ------------- -------------
or the same as directed graph:

Correct: "marital_status(J J ADAMS) is married"
Incorrect: "marital_status(J J ADAMS) is divorced"


Discussion of Database Systems

Example: Relations          R1: Person
-------- ------------- -------------
 J J ADAMS  025  male  married
-------- ------------- -------------
 P K BROWN  032  male  single
-------- ------------- -------------
 
.
.
.
 
.
.
.
 
.
.
.
 
.
.
.
-------- ------------- -------------

R2: Employment
-------- ------------- -------------
 XYZ  J J ADAMS
-------- ------------- -------------
 XYZ  P K BROWN
-------- ------------- -------------
 ABC  P K BROWN
-------- ------------- -------------
 
.
.
.
 
.
.
.
-------- ------------- -------------
           R3: Parent/child
-------- ------------- -------------
 J J ADAMS  Sally
-------- ------------- -------------
 J J ADAMS  Richard
-------- ------------- -------------
 J J ADAMS  Bob
-------- ------------- -------------
 
.
.
.
 
.
.
.
-------- ------------- -------------

Properties of trad. database systems:

Knowledge Representation in Semantic Nets

Example:

Correct:
height of Three-Finger Brown is 195cm
Incorrect:
height of Three-Finger Brown is 178cm


Semantic Nets (Cont'd)

Properties of semantic nets:


Frames

Frames: A frame consists of a collection of slots which can be filled by values or pointers to other frames.

Meaning of "child's birthday party" poorly approximated by definition like "a party assembled to celebrate a birthday" with "party" defined as "people assembled for a celebration".            Children know more plus default assignments:

-------- ------------- -------------
 Child's Birthday Party
-------- ------------- -------------
-------- ------------- -------------
 Dress:  Sunday-Best
-------- ------------- -------------
 Present:  Must please host.
   Must be bought and gift-wrapped.
-------- ------------- -------------
 Games:  Hide and seek. Pin tail on donkey.
-------- ------------- -------------
 Decor:  Balloons. Favours. Crepe-paper
-------- ------------- -------------
 Party-meal:  Cake. Ice-cream. Soda. Hotdogs
-------- ------------- -------------
 Cake:  Candles. Blow-out. Wish.
   Sing Birthday Song.
-------- ------------- -------------
 Ice-cream:  Standard three-flavour
-------- ------------- -------------


Why Frames?

Quotes from "A Framework for Representing Knowledge" [Minsky81]

"It seems to me that the ingredients of most theories [...] have been on the whole too minute, local, and unstructured to account [...] common-sense thought"

"When one encounters a new situation [...], one selects from memory a structure called a frame. This is a remembered framework to be adapted to fit reality by changing details as necessary.

A frame is a data-structure for representing a stereotyped situation, like [...] going to a child's birthday party. Attached to each frame are several kinds of information. Some of this information is about how to use the frame. Some is about what one can expect to happen next. Some is about what to do if these expectations are not confirmed."


Frames - What and Why?

[Minsky, 1981]:
A Frame is a collection of questions to be asked about a hypothetical situation: it specifies issues to be raised and methods to be used in dealing with them.

To understand a situation, questions like: