1 Knowledge Representation (continue)

2 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

3 Semantic networks animal skin eat does has ostrich isa robin seagull isa bird fly wings feathers has can has canarysing isa does can’t isa Collins & Quillian (1969)

4 Semantic Net Basic Constructs –Node-Object, Concept –Links-Relation property inheritance TweetyRobinBird Wings isa has-part

5 Nodes and Arcs Arcs define binary relations which hold between objects denoted by the nodes. SueJohn5 Max34 motherage father age wife husband mother (john, sue) age (john, 5) wife (sue, max) age (max, 34) …

6 Non-binary relations We can represent the generic give event as a relation involving three things: –A giver –A recipient –An object MaryGIVEJohn book recipientgiver object

7 Advantages of Semantic nets Easy to visualize Related knowledge is easily clustered. Efficient in space requirements –Objects represented only once –Relationships handled by pointers

8 Problems of Semantic Net 1. Different people use different nets to represent the same thing. “John is taller than Bill.” JohnBill John Bill H1NumberH2 Is-taller height isa height greater-than

9 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 Tom Father-of

10 Representing General Statements

11 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

12 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

13 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

14 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.

15 Example 1. It is winterOUT 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))?

16 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.

17 Example Nodes 1. Not OK(SL()(2))IN 2. She likes him (SL(3)(4)) (SL(5,6)(4,7)) OUT 3. He is richOUT 4. He has a problem (SL(8)(9)) (SL(10)()) OUT 5. He is healthy OUT 6. Kind OUT 7. The eldest sonOUT 8. Older than 35OUT 9. Exception(SL(11)())OUT 10. MarriedOUT 11. LoveOUT

18 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 richOUT 4. He has a problem (SL(8)(9)) (SL(10)()) OUT 5. He is healthy(SL()())IN 6. Kind (SL()())IN 7. The eldest sonOUT 8. Older than 35OUT 9. Exception(SL(11)())OUT 10. MarriedOUT 11. LoveOUT Currnet belief: He is healthy and kind She likes him --- OK

19 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 richOUT 4. He has a problem (SL(8)(9)) (SL(10)()) OUT -> IN 5. He is healthyIN 6. Kind IN 7. The eldest sonOUT 8. Older than 35(SL()())IN 9. Exception(SL(11)())OUT 10. MarriedOUT 11. LoveOUT

20 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 richOUT 4. He has a problem (SL(8)(9)) (SL(10)()) IN -> OUT 5. He is healthyIN 6. Kind IN 7. The eldest sonOUT 8. Older than 35(SL()())IN 9. Exception(SL(11)())OUT -> IN 10. MarriedOUT 11. Love(SL()())IN -- So they married, and happily there after …