06 knowledge representation (us)

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Artificial Intelligence

Knowledge representation

Fall 2008

professor: Luigi Ceccaroni


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Introduction

Knowledge engineers and system

analysts need to bring knowledge fort

and make it e!plicit" #Why?$

%ey display te implicit knowledge about

a sub&ect in a form tat programmers can

encode in algoritms and data structures"

%o make te idden knowledge accessible

to computers' knowledge-based

systemsand object-oriented systems

are needed" 2


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(

Introduction

Knowledge)based and ob&ect)oriented systems

are built around declarative languages:

Forms of e!pression closer to uman languages

*uc systems elp to e!press te knowledge ina form tat bot umans and computers can

understand"

%is part of te course is about knowledge)baseanalysis and design:

%o analy+e knowledge about te real world and map

it to a computable form

(


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,

Logic' ontology and

computation Knowledge representation #K-$ is an

interdisciplinary sub&ect tat applies

teories and tecni.ues from tree fields:

Logicpro/ides te formal structureand

rules of inference"

Ontologydefines te kinds of things that

existin te application domain"Computationsupports te applicationstat

distinguis K- from pure pilosopy"

,


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1rinciples of knowledge

representation Knowledge engineering is te application

of logic and ontology to te task of

building computable modelsof some

domainfor some purpose"

In 33(' tree e!perts in K-' 4a/is'

*crobe and *+olo/its' wrote a critical

re/iew and analysis of te state of te art: Fi/e basic principles about knowledge

representations#K-s$ and teir role in

artificial intelligence


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5at is a

knowledge representation6" A surrogate

7 Imperfect surrogates mean incorrect inferences areine/itable

2" A set of ontological commitments

7 Commitment begins wit te earliest coices7 %e commitments accumulate in layers7 -eminder: a K- is not a data structure

(" A fragmentary theory of intelligent reasoning

7 5at is intelligent reasoning67 5ic inferences are sanctioned67 5ic inferences are recommended6

," A medium for efficient computation

" A medium of human expression


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A K- is a surrogate

4escription of someting else

Abstract' simplified /iew of a domain

*ymbolic structure wit formal symbol)manipulating

rules -ules are based only on te syntactic form of te

representation

-e.uires specification of mapping to intendedreferent: an interpretation

Contains simplifying assumptions and inaccuracies

*usceptible to supporting incorrect reasoningresults


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A K- is a set of ontological

commitments 5at to consider in tinking about a

world: concepts' relations' ob&ects !ample: representing an electric circuit

9Lumped element model;Components wit connections between tem

;Signals flowing instantaneously along te connections

9lectrodynamics model

;Signals propagating at finite speeds;Locations of and distances between components

;Components troug wic electromagnetic waves flow

K- is not about data structures


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A K- is a set of ontological

commitmentsAn ontological commitment is an agreement

to use a /ocabulary #i"e"' ask .ueries and

make assertions$ in a way tat is consistent

#but not complete$ wit respect to teteory specified by an ontology" 5e build

agents tat commit to ontologies" 5e

design ontologies so we can sareknowledge wit and among tese agents"

%om


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A K- is a fragmentary teory of

intelligent reasoning It pro/ides different strategies for

reasoning"

%ese strategies can be used by umans and

computers"

It sanctions a set of inferences"

95at can we infer from wat we know6

It recommends a set of inferences"

95at ougt we to infer from wat we know6


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A K- is a medium for efficient

computation -easoning in macines is a computational

process:

=ot te procedural and te declarati/e

approaces can be transformed to a

computable form"

Computational efficiency is a central

design goal" !pressi/ity and tractability of reasoning

are traded off"


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A K- is a medium of uman

e!pression >ow useful is it as a medium of e!pression6

>ow general is it6

>ow precise is it6

For wat tasks does it pro/ide e!pressi/e ade.uacy6

>ow useful is it as a medium of communication6

Can we easily 9talk or tink in te representation

language6

5at kinds of tings are easily said in te language6

5at kinds of tings are so difficult to say in te

language as to be pragmatically impossible6


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K-s /s" data bases

=ot 9represent knowledge"

*tandard data bases do notcontain: dis&unctions #e"g"' 9%e ball is eiter red or blue"$

.uantifiers #e"g"' 9/ery person as two parents"$

4ata base scema pro/ide some .uantified information 4educti/e data bases include implications

4ata base researc concerns: fficient access and management of large distributed data

bases

Concurrent updating

K- researc concerns: !pressi/ity

ffecti/e reasoning


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,

5at is a knowledge base

#K=$6An informal term for a collection of

information tat includes an ontology as

one component"

=esides an ontology' a K= may contain

information specified in a declarati/e

language suc as logic or e!pert)system

rules" It may also include unstructured or

unformali+ed information e!pressed in

natural language or procedural code" ,


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Issues in K- researc

5at knowledge needs to be represented

to answer gi/en .uestions6

>ow is incomplete or noisy information

represented6

>ow is .ualitati/e or abstracted knowledge

represented6

>ow can knowledge be encoded so tat it

is reusable6

>ow are assumptions represented and

reasoned wit6


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>ow can knowledge be reformulated for a

gi/en purpose6

>ow can effecti/e automatic reasoning be

done wit large)scale knowledge bases6

>ow can computer)interpretable

knowledge be e!tracted from documents6

>ow can knowledge from multiple sources

be combined and used6


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%is is a world were massi/e amounts ofdata and applied matematics replace e/eryoter tool:

?ut wit e/ery teory of uman bea/ior' fromlinguistics to sociology" Forget ta!onomy' ontology' and psycology"

5o knows wy people do wat tey do6

%e point is tey do it' and we can track andmeasure it wit unprecedented fidelity" 5it enoug data' te numbers speak for

temsel/es"

Cris Anderson


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>istorical background

%e words knowledgeand representationa/e

pro/oked pilosopical contro/ersies for o/er

200 years"

00 ="C": *ocrates claims to know /ery little' ifanyting"

>e destroyed te self)satisfaction of people wo

claimed to a/e knowledge of fundamental

sub&ects like: %rut

=eauty

@irtue

ustice8


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For is impiety in .uestioning cerised

beliefs' *ocrates was condemned to deat as

a corrupter of te morals Atenian yout"

*ocratesB student 1lato establised te sub&ectof epistemology:

te study of te nature of knowledge and its

&ustification

1latoBs student Aristotle sifted te empasis

of pilosopy from te nature of knowledge to

te less contro/ersial but more practical

problem of representing knowledge" 3


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AristotleBs work resulted in an encyclopedic

compilation of te knowledge of is day"

=ut before e could compile tat

knowledge' e ad to in/ent te words for

representing it"

>e establised te initial terminology and

defined te scope of logic' pysics'metapysics' biology' psycology'

linguistics' politics' etics' retoric and

economics" 20


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%erms tat Aristotle coined or adopted a/e

become te core of todayBs international

tecnical /ocabulary:

category

metaphor

hypothesis

quantity

quality

species

noun

""" and ten artificial intelligence arri/ed" 2


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arly istory of K- #0Bs ) D0Bs$

?rigins

1roblem sol/ing work primarily at CE and EI%

Gatural language understanding

Eany ad hocformalisms 91rocedural /s" 9declarati/e knowledge

1rocedural: functions' rules' con/entional

programming languages

4eclarati/e: logic' 1rolog

Go formal semantics


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7 *emantic nets; nstructured node)link graps

; Go semantics to support interpretation

; Go a!ioms to support reasoning

; -eference:

95atBs in a Link: Foundations for *emantic GetsH 5oods' 5" A"

In -epresentation and nderstanding: *tudies in Cogniti/e *cienceHedited by 4" =obrow and A" CollinsH Academic 1ressH 3D"

merging paradigms

#D0Bs ) 80Bs$


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merging paradigms

#D0Bs ) 80Bs$7 Frames

; *tructured semantic nets

; ?b&ect)oriented descriptions

; 1rototypes

; Class)subclass ta!onomies

; -eference:

9A Framework for -epresenting Knowledge E"Einsky

Mind DesignH " >augeland' editorH EI% 1ressH 38"


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!ample: Frames: class)

subclass ta!onomy


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!ample: Frames: Classframe


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!ample: Frames: nstance

frame


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merging paradigms

#D0Bs ) 80Bs$7 1roduction rule systems

; If)ten inference rules7 If #warning)ligt on$ ten #engine o/ereating$

7 If #warning)ligt on$ ten ##engine o/ereating$0"3$

; *ituation)action rules7 If #warning)ligt on$ ten #turn)off engine$

; >ybrid procedural)declarati/e representation; =asis for e!pert systems


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merging paradigms

#D0Bs ) 80Bs$7 ualitati/e pysics

; -epresenting and reasoning:

7 5it incomplete knowledge

7 About pysical mecanisms

; ualitati/e descriptions

7 Capture distinctions tat make an important

.ualitati/e difference and ignores oters

7 Aggregate /alues tat a/e no .ualitati/e

difference


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merging paradigms

#D0Bs ) 80Bs$7 *ymbolic Logic

; 1rimarily first)order logic9/erybody lo/es somebody sometime"

#forall 6p #implies #1erson 6p$ #e!ists #6p2 6t$ #and #1erson 6p2$

#%ime 6t$

#Lo/es 6p 6p2 6t$$$$$

; -esolution teorem pro/ing


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K- in te 30Bs and 00Bs

7 4eclarati/e representations; asier to cange

; Eulti)use

; !tendable by reasoning

; Accessible for introspection

7 Formal semantics; 4efines wat te representation means

; *pecifies correct reasoning

; Allows comparison of representationsJalgoritms7 K- rooted in te study of logics

; temporal' conte!t' modal' default' nonmonotonic"""

7 -igorous teoretical analysis

06 knowledge representation (us)

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