COMP219: Artificial Intelligence. Lecture 17: Semantic Networks

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1 COMP219: Artificial Intelligence Lecture 17: Semantic Networks 1

2 Overview Last time Rules as a KR scheme; forward vs backward chaining Today Another approach to knowledge representation Structured objects: semantic nets Notation Extended example Learning outcomes covered today: Distinguish the characteristics, and advantages and disadvantages, of the major knowledge representation paradigms that have been used in AI, such as production rules, semantic networks, propositional logic and first-order logic; Solve simple knowledge-based problems using the AI representations studied; 2

3 Structured Objects Structured objects are Knowledge representation formalisms whose components are essentially similar to the nodes and arcs found in graphs In contrast to production rules and formal logic 3

Semantic Networks Charles S Peirce 1909 existential graphs Quillian in 1968 semantic memory as a model for human memory; reasonable view of how semantic information is stored by humans

4 Semantic Networks Charles S Peirce 1909 existential graphs Quillian in 1968 semantic memory as a model for human memory; reasonable view of how semantic information is stored by humans associative reasoning (via links) Semantic net is a labelled graph nodes in graph represent objects, concepts, or situations/events arcs in graph represent relationships between these things 4

5 Semantic Networks Relationships Concepts concepts Individuals individuals 5

6 Important Arc Types Subset X is a kind of Y Penguin subset Bird: Concept to Concept Member X is a Y: X is an instance of Y Opus member Penguin: Individual to Concept R-relation X relation-name Y Opus is a friend of Bill; Lou is a parent of Ian Individual to Individual 6

7 Inheritance Inheritance is one of the main kinds of reasoning done in semantic nets The subset relation is often used to link a class and its superclass Some links (e.g. legs) are inherited along subset paths Many variants of semantic nets semantics can be relatively informal or very formal What's in a Link: Foundations for Semantic Networks (Woods, 1975) 7

8 Example 8

9 Example Bill has four legs 9

10 Example Bill has four legs 10

11 Example Bill has four legs 11

12 Example Bill has four legs 12

13 Example Bill has four legs 13

14 Example Bill has four legs Opus is a Bird 14

15 Example Bill has four legs Opus is a Bird 15

16 Example Bill has four legs Opus is a Bird 16

17 Example Bill has four legs Opus is a Bird 17

18 Example Bill has four legs Opus is a Bird Opus walks 18

19 Multiple Inheritance A node can have any number of superclasses that contain it, enabling a node to inherit properties from multiple parent nodes and their ancestors in the network. It can cause conflicting inheritance Nixon Diamond: 19

20 Problems with Semantic Nets Binary relations are easy to represent Others are harder Example: Opus brings tequila to the party where who Party Brings Opus what Tequila 20

21 Exercise Suppose we have the information Bill brings whiskey to the party. How could we extend the semantic network to include this information? Can you see any problems with the reasoning in the example once we introduce this information? 21

22 Binary Relations Any relation can be rewritten as a set of binary relations Bringing-1(Opus,tequilla,party) Bringing-2(Bill,whiskey,party) Make the event a thing and make one binary relation per role who(bringing-1,opus); who(bringing-2,bill) what(bringing-1,tequila); what(bringing-2,whiskey) where(bringing-1,party); where(bringing-2,party) 22

23 Binary Relations Any relation can be rewritten Turn relation as a set into a of thing: binary relations 'thingification' (McCarthy) more common: Bringing-1(Opus,tequilla,party) reification from Latin (res) Bringing-2(Bill,whiskey,party) Make the event a thing and make one binary relation per role who(bringing-1,opus); who(bringing-2,bill) what(bringing-1,tequila); what(bringing-2,whiskey) where(bringing-1,party); where(bringing-2,party) 23

24 Now we can see who brought what where Bringing 1 what tequila Party who Opus where Bringing 2 what whiskey who Bill 24

25 Other Problems are Harder Negation Opus and Dirk are not friends Can just assume an absence of a link Cancellation/Exception Property inherited from a distant superclass cancelled at a lower level Birds fly, penguins don t Disjunction Opus either drinks tea or coffee Quantification every dog has bitten a postman every dog has bitten every postman 25

26 Advantages of Semantic Nets Easy to visualise Flexible: relationships can be arbitrarily defined by the knowledge engineer Formal definitions of semantic networks have been developed Related knowledge is easily clustered Efficient in space requirements Objects represented only once Inference reduced to search 26

27 Disadvantages of Semantic Nets Inheritance (particularly from multiple sources and when exceptions in inheritance are required) can cause problems No standards about node and arc types, and semantics might not be quite clear (what does IS-A mea?n) Limited expressiveness: may require a number of specially coded procedures ('procedural attachment') The above problems make it difficult to verify and validate the systems share knowledge reuse knowledge acquire knowledge methodically 27

28 wikipedia The Story of Othello Othello was a general who was married to Desdemona Iago was a captain who was married to Emilia; he hated Othello Iago told Othello lies about Desdemona Othello killed Desdemona with a pillow. He felt remorse and killed himself with a dagger 28

29 Othello was a general who was married to Desdemona rank general Othello Married to Desdem ona 29

30 Iago was a captain who was married to Emilia; he hated Othello rank general Othello Married to Desdem ona hated Iago Married to Emilia rank captain 30

31 Iago told Othello lies about Desdemona rank general Othello hated Married to deceived Lying-1 about Desdem ona liar Iago Married to Emilia rank captain 31

32 dagger weapon motive remorse general killed Killing-2 Killing-1 motive rank killer killer killed jealousy captain Othello hated Married to deceived Lying-1 about Desdem ona weapon Pillow rank Iago liar Married to Emilia Othello killed Desdemona with a pillow. He felt remorse and killed himself with a dagger 32

33 Prolog Organised by Relations marriedto(husband,wife). marriedto(x,y):-marriedto(y,x). rank(soldier,rank). male(person). alive(person). killing(killer,killed,weapon,motive). motiveforkilling(person,motive):- killing(person,_,_,motive). And so on 33

34 Manipulating the Knowledge So far we have represented the knowledge in a variety of ways We also need to manipulate the knowledge This can be done in a variety of ways 34

35 dagger weapon motive remorse Spreading Activation general killed Killing-2 Killing-1 motive rank killer killer killed jealousy captain Othello hated Married to deceived Lying-1 about Desdem ona weapon Pillow rank Iago liar Married to Emilia What do the pillow and the dagger have in common? 35

36 dagger 1 weapon motive remorse Spreading Activation general killed Killing-2 Killing-1 motive rank killer killer killed jealousy captain Othello hated Married to deceived Lying-1 about Desdem ona weapon 1 Pillow rank Iago liar Married to Emilia What do the pillow and the dagger have in common? 36

37 dagger 1 weapon motive 2 remorse Spreading Activation general killed Killing-2 Killing-1 motive rank killer killer killed jealousy captain Othello hated Married to deceived Lying-1 about Desdem ona weapon 1 Pillow rank Iago liar Married to Emilia What do the pillow and the dagger have in common? 37

38 general dagger 1 weapon killed motive 2 Killing-2 remorse 2 Killing-1 Spreading Activation motive rank killer killer killed jealousy captain Othello hated Married to deceived Lying-1 about Desdem ona weapon 1 Pillow rank Iago liar Married to Emilia What do the pillow and the dagger have in common? 38

39 general captain rank dagger 1 motive weapon 2 Killing-2 killed killer rank killer 3 Married to Othello deceived Lying-1 hated liar Married to Iago 3 remorse about killed Desdem ona Emilia 2 Killing-1 weapon 1 Pillow Spreading Activation motive jealousy What do the pillow and the dagger have in common? 39

40 general captain rank dagger 1 motive weapon 2 Killing-2 killed killer rank killer 3 3 Married to Othello deceived Lying-1 hated liar Married to Iago 3 remorse about killed 3 Desdem ona Emilia 2 Killing-1 weapon 1 Pillow Spreading Activation motive 3 jealousy What do the pillow and the dagger have in common? 40

41 general captain rank dagger 1 motive weapon 2 Killing-2 killed killer rank killer 3 3 Married to Othello deceived Lying-1 hated liar Married to Iago 3 remorse about killed 3 Desdem ona Emilia 2 Killing-1 weapon 1 Pillow Spreading Activation motive 3 jealousy What do the pillow and the dagger have in common? Weapons used by Othello in killings 41

42 Using Rules IF (?X is-a killing) AND (?X killed?y) THEN REMOVE (?Y alive T) AND ADD (?Y alive F). IF create(killing,?x,?y) THEN execute(?x.weapon) AND execute(?x.motive) AND put(?y.alive,f). Or we can use clauses for Prolog alive(x,false):-killing(_,x,_,_). 42

43 Frames Development of semantic nets Desire to exploit the powerful mechanism of inheritance Observation: things of a given type participate in the same set of relationships A lot of information is available by default it is the exceptions that are interesting 43

Frames Frames - semantic net with properties and methods Devised by Marvin Minsky, 1974.

44 Frames Frames - semantic net with properties and methods Devised by Marvin Minsky, Incorporates certain valuable human thinking characteristics: Expectations, assumptions, stereotypes, exceptions. The essence of this form of knowledge is that we represent the typical case and exceptions, rather than give definitions. Hierarchical structure, similar to class hierarchies. 44

45 Problems with Frames & Semantic Nets Useful for representing certain sorts of knowledge e.g., inheritance But node and edge types can be ad hoc. no clear meaning, or semantics. Inheritance reasoning is very easy, but more general reasoning is difficult to define often special purpose. 45

46 Developments Many of the ideas of frames are now expressed in ontologies (see next lecture) Frame system + procedures for retrieving and manipulating knowledge = Object System AI research influenced the development of Object Oriented Programming, which has become a standard paradigm good example of how AI contributes to mainstream computing 46

47 Agents Agents can be seen as a development from OO programming: Agents don t wait for messages: they proactively poll the environment to find new information. Agents decide whether to respond to messages. The elements of proactivity and autonomy make them part of AI. 47

48 Summary Semantic networks were a popular method of structuring information In recent years people have attempted to be more principled and formal Simply working on special cases and limited domains is no longer enough Next we will consider these developments in the context of ontologies and logic-based approaches Structured objects developed into OO programming, now a conventional technique Next time Expert systems and ontologies 48

COMP219: Artificial Intelligence. Lecture 17: Semantic Networks

COMP219: Artificial Intelligence Lecture 17: Semantic Networks 1 Overview Last time Rules as a KR scheme; forward vs backward chaining Today Another approach to knowledge representation Structured objects: