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Knowledge Representation

Representing Common Sense Knowledge

MSc Decision Support Notes 2

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How to represent advice? Memory? Plans? Common Sense?

• What’s this Or this

• How many legs does the man have? The tortoise?– How do you know? Is it in the picture?

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How to answer a question?

• Help - my laser printer doesn’t work?– Is it plugged in? Is the power on at the wall? – Is it connected to the computer?– Does it have paper?– Does it print a test sheet?– Have you turned everything off, turned on the printers, then

rebooted the computer?

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How to understand a story

• John went into the restaurant. He sat down. When the waiter came he ordered a pizza. He waited for forty minutes. Finally, he got up and left. The waiter rushed after him very angry. John was also very angry.

– Why was John angry?Why was the waiter angry?

– How do you know?

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Two parts to most knowledge representations

• Knowledge of the things in the world and how the relate– The “Ontology” or “Static knowledge”

• Semantic Nets, Frames, Description Logics, Conceptual Graphs,…– BEWARE “The ‘O’ word”

“Ontology” means different things to different people

– Declarative• Logic like

• Knowledge of what to do and how– The “Operational knowledge”

• Rules, Problem Solving Methods, Methods, Operations…

– Procedural• Program like or Condition-Action rules

– Usually “Heuristic”

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NB: Real “Expertise” is different

• Tacit, automatic, semi-conscious

• Experts may reconstruct an explanation but can rarely explain what they do– See Johnson papers on “Links”

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Classic Knowledge Based Systems

• Knowledge base– Typically in “Frames”

• The problem solving methods or “Heuristics”– Typically in “Rules”

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by the way… vocabulary…“Algorithms” & “Heuristics”

• Algorithms – methods guaranteed to do something – Guaranteed to get an answer– Example: Long division, manual extraction of a square root, ‘exhaustive

chess algorithm’– Never need more than one for any one problem– May take a very long (age of universen) time

• We will return to topic when we discuss ontologies, OWL, and description logics

• Heuristics – rules of thumb for doing things– Often work, but may not– A good heuristic either gets the right answer or fails– Usually apply several to each problem to give a high probability that one

will work– Usually chosen to be relatively quick

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The knowledge base

• How to represent the things in the world and their relationships– Or more accurately:

our conceptualisation of the things in the world & their relationships

– The starting metaphor was to explain our memory

– How might we represent the knowledge we need to understand the restaurant story?

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Memory and Association

CustomerRestaurant

Food

Money

Waiter

Hunger

Anger

Boredom

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Semantic Net - Label the arcs

Hunger

Anger

Boredom

CustomerRestaurant

Food

Money

Waiter

has

has Pays to

Works

for

causes

causes

has

has

Serves food

Takes order

has

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Add the classes

CustomerRestaurant

Food

Money

Waiter

Hunger

Anger

Boredom

hasPays to

Works for

has

causes

causes

has

has

Serves food

Takes order

has

Person

Emotion

Thing

Organisa-tion

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A Real Example: Use cases from UK Drug Messages

14From Evans & Patel, Cog Sci in Biomedicine pg 72

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It’s getting messy!What do all those lines mean?

Can we find a better way to write it down?

• Frames are one systematic way to write down semantic nets– Formalised by the language KL-ONE

• Re-formalised as “Description Logics”– Being restandardised as OIL: (Ontology Inference Layer) - new proposed

interchange language DAML+OIL or “OWL”www.w3.org/2001/sw/WebOnt/http://oiled.man.ac.uk/www.ontoknowledge.org/oil (click here)

• Other more formal ways are:– Conceptual graphs (click here)

• A complete alternative notation for logic– John Sowa

http://www.bestweb.net/~sowa/direct/jfsbio.htm

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Frames

• May be regarded as– A knowledge representation formalism– A way of writing down semantic networks– A set of data structures– A housekeeping trick

• No real standards– Grew up informally– Much confusion of vocabulary and notation

• Hence the development of detailed standards with different names

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Basic ideas

• Types/ classes– The categories of things that are:

• Analogous to setsBest expressed as plurals but usually written as singular

– Mammal, Dog, Bottle

• Instances / objects– The things themselves

• Analogous to members of setsbest expressed as singular, or with ‘the’ or ‘this’ or ‘these’, etc.

– Fido, This bottle of milk

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Basic ideas

• A hierarchy of classes – LivingThings

Animals Mammals Dogs Golden Retrievers Sansue Golden Retrievers Sansue Golden Retrievers from the Phoenix line ...

• Instances• Mia is-instance-of Sansu Golden Retrievers• Mammals is-kind-of of Animals

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Basic ideas

• Types/ classes• Subclasses linked to classes by “is-kind-of” / “specialises” / “is

subsumed by” “ako” (“is kind of”)

• Classes linked to subclasses by “has-kind” / “subsumes” / “is generalisation of”

• Dog is-kind-of Mammal is-kind-of Vertebrate is-kind-of Animal• Animal subsumes Vertebrate subsumes Mammal subsumes Dog

• Individuals/ Instances / objects• Instances linked to types by “is-instance-of”

– BEWARE “is-a” may mean “is-instance-of” or “is-kind-of” depending on the system!

• Fido is-instance-of Dog; John is-instance-of Man; • This bottle is-instance-of Bottle

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Graphic NotationArrows (should) always point up

Animal

Mammal

Dog

Fido

•Conventions

•Arrows always point up

•Open arrows for is-kind-of

•Closed arrows for is-instance-of

•In this course

•Round boxes for Types/Classes

•Ovals for instances/objects

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Adding Slots

• LivingThings mode-of-reproduction: ? Animals source-of-food: ? covering: ? means-of-feeding-young: ? Mammals: means-of-feeding-young: milk covering: fur normal-body-termperature: ?

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Inheritance

• What is true of the superclass– is (generally) true of the subclass

• In many frame systems, the ‘default’ values can be over-ridden– In description logics and OIL, faults do not exist as such

• Vocabulary– “Defeasible”

• can be over-ridden– defaults

– “Indefeasible”• not defeasible

– cannot be over-ridden

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Inheritance: All Slots are inherited• LivingThing

mode-of-reproduction: ? Animal (mode of reproduction: ?)

source-of-food: ? covering: ? means-of-feeding-young: ? Mammal: (mode of reproduction: ?)

(source-of-food: ?)(means-of-feeding-young: ?)

(covering: ?)

normal-body-termperature: ?Dog

...

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Inheritance: Default values are inherited

• Mammal: (mode of reproduction: live-birth)

(source-of-food: ?)(means-of-feeding-young: milk)

(covering: fur)

normal-body-termperature: ?Dog

(mode of reproduction: live-birth)(source-of-food: ?)(means-of-feeding-young: milk)

(covering: fur) (normal-body-termperature: ?)

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Inheritance: Default values can be overridden

• Mammal: (mode of reproduction: live-birth)

(source-of-food: ?)(means-of-feeding-young: milk)

(covering: fur)

normal-body-termperature:?Platypus

(mode of reproduction: lays-eggs)(source-of-food: ?)(means-of-feeding-young: milk)

(covering: fur) (normal-body-termperature: ?)

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Single and Multiple Inheritance

Person

Quaker Republican

Nixon

Nixon is an instance of both Quaker and Republican

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Multiple Inheritance with DefaultsThe ‘Nixon Diamond’

Person

Quaker Republican

Nixon

:pacifist true :pacifist false

Is Nixon a pacifist?

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Multiple inheritance and defaults

• Each seem a minor change

• Each alone works

• Together they spell disaster– Computational consequences are often unintuitive

• In classification they are rarely intuitive

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Advantages of Frames

• An orderly easy-to-understand structure

• Inheritance helps to keep knowledge modular

• Efficient inference– If a single hierarchy

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Problems with frames

• Negation cannot be represented– “Jim does not have pneumonia”

• Disjunction cannot be represented

• Semantics ambiguous– Woods: What’s in a link– Brachman: What IS-A is and IS-A isn’t

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Frames and Object Oriented Programming

• Frame systems were one of the ancestors of OO programming– As used, slots are very similar to instance variables– Default values can be implemented as class variables– Many frame systems allow methods in slots

• Analogous to methods in OO programming

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Exercises

• Start up PROTÉGÉ

• Work through the tutorial

• Load the Newspapers example

• Create a new kind of “Article” - “Editorial” - and give it a new slot “Stance”

• Create a new kind of “Article” - “Guest editorial” - and give it a new slots “Own organisation” and “political orientation”

• Find the Biological Process (malaria) model on the web from http://protege.stanford.edu and explore it