1 knowledge representation (continue). 2 knowledge representation logic isn’t the only method of...

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Knowledge Representation(continue)

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Knowledge Representation

  Logic isn’t the only method of representing knowledge.

  There are other methods which are less general, but more natural, and arguably easier to work with:– Semantic Nets– Frames– Conceptual Dependency– Scripts

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Semantic networksSemantic networks

animalskin

eatdoes

has

ostrich

isa

robin

seagull

isa

isa

bird

fly wings

feathershas

can has

canary sing

isadoes

can’t

isa

Collins & Quillian(1969)

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Semantic Net

  Basic Constructs– Node-Object, Concept– Links-Relation

property inheritance

Tweety Robin Bird

Wings Wings Wings

isa

isa

has-part

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Nodes and Arcs

  Arcs define binary relations which hold between objects denoted by the nodes.

Sue John 5

Max34

mother age

fatherage

wifehusband

mother (john, sue)age (john, 5)wife (sue, max)age (max, 34)…

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Non-binary relations

  We can represent the generic give event as a relation involving three things: – A giver– A recipient– An object

Mary GIVE John

book

recipient giver

object

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Advantages of Semantic nets

• Easy to visualize

• Related knowledge is easily clustered.

• Efficient in space requirements– Objects represented only once– Relationships handled by pointers

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Problems of Semantic Net

1. Different people use different nets to represent the same thing.

“John is taller than Bill.”

John Bill

John Bill

H1 Number H2

Is-taller

height

isa isa

height

greater-than

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Problems of Semantic Net

2. Same Net interpreted differently by different person.

3. Quantification and intentional concepts are hard to represent.

– Some birds fly– All the birds sing some of the songs– Some of the birds sing all the songs– Mike thinks that Jane’s belief that Bernard will like their new home is false.

Jack TomFather-of

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Representing General Statements

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Types of Reasoning

• Monotonic reasoning

- new piece of knowledge cannot reduce the set of what is known

• Non-monotonic reasoning

- new piece of knowledge may contradict with what is known

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TMS

• Truth Maintenance System - Doyle

• Information pieces are linked together by their justifications.

• Dependency-directed backtracking

• Basic Data Structure– Node: belief

– Justification: reason to believe

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TMS

• 2 states of node– IN – current belief– OUT – not believed (believed to be not true)

• A node is assigned a justification set• A node is IN iff there is at least one valid

justification• A node is OUT iff there is no valid

justification

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SL justification

(SL (list of IN-nodes)(list of OUT-nodes))– SL-justification is valid if all the nodes in the IN-node list

are currently IN, and those in the OUT-node list are OUT.

Statement-1: (SL (x)(y))

  Meaning:– If x is believed and y is not believed, the statement-1 is

believed.

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Example

1. It is winter OUT

2. It is cold (SL(1)(3)) OUT 3. It is warm (SL(4)(2)) IN 4. It is summer (SL()(1)) IN

It is winter. 1. It is winter (SL()()) IN 2. It is cold (SL(1)(3)) ? 3. It is warm (SL(4)(2)) ? 4. It is summer (SL()(1)) ?

It is warm outside. 1. It is winter (SL()()) IN 2. It is cold (SL(1)(3)) ? 3. It is warm (SL(4)(2)) (SL()()) IN 4. It is summer (SL()(1)) ?

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Example

  This is how Mary likes to see as her marriage partner.– Not OK unless she really likes him.– She likes a rich man as long as he doesn’t have a

problem.– She likes a man if he is healthy and kind as long as he

does not have a problem and is not the eldest son.– A man is problematic if he is older than 35 unless he is

exceptional.– Married man is problematic– Love is an exception.

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Example

  Nodes 1. Not OK (SL()(2)) IN

2. She likes him (SL(3)(4)) (SL(5,6)(4,7)) OUT

3. He is rich OUT

4. He has a problem (SL(8)(9)) (SL(10)()) OUT

5. He is healthy OUT

6. Kind OUT

7. The eldest son OUT

8. Older than 35 OUT

9. Exception (SL(11)()) OUT

10. Married OUT

11. Love OUT

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Example

  He looks healthy and kind 1. Not OK (SL()(2)) IN -> OUT

2. She likes him (SL(3)(4)) (SL(5,6)(4,7)) OUT -> IN

3. He is rich OUT

4. He has a problem (SL(8)(9)) (SL(10)()) OUT

5. He is healthy (SL()()) IN

6. Kind (SL()()) IN

7. The eldest son OUT

8. Older than 35 OUT

9. Exception (SL(11)()) OUT

10. Married OUT

11. Love OUT

Currnet belief:

He is healthy and kind

She likes him --- OK

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Example

  His age is 38! 1. Not OK (SL()(2)) OUT -> IN

2. She likes him (SL(3)(4)) (SL(5,6)(4,7)) IN -> OUT

3. He is rich OUT

4. He has a problem (SL(8)(9)) (SL(10)()) OUT -> IN

5. He is healthy IN

6. Kind IN

7. The eldest son OUT

8. Older than 35 (SL()()) IN

9. Exception (SL(11)()) OUT

10. Married OUT

11. Love OUT

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Example

  Mary finds herself that she is in love with him. 1. Not OK (SL ()(2)) IN -> OUT

2. She likes him (SL(3)(4)) (SL(5,6)(4,7)) OUT -> IN

3. He is rich OUT

4. He has a problem (SL(8)(9)) (SL(10)()) IN -> OUT

5. He is healthy IN

6. Kind IN

7. The eldest son OUT

8. Older than 35 (SL()()) IN

9. Exception (SL(11)()) OUT -> IN

10. Married OUT

11. Love (SL()()) IN

-- So they married, and happily there after …