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