Chapter 5 Knowledge Representation,Xiu-jun GONG (Ph. D) School of Computer Science and Technology, Tianjin University gongxjtju.edu.cn http:/cs.tju.edu.cn/faculties/gongxj/course/ai/,Outline,Knowledge & Knowledge representation Methodology for KR Logic Production System Semantic Net Frame Script Object-Oriented Summary,Knowledge,What is Knowledge ?,Information,Knowledge,Knowledge = FactsRulesControl Strategy +(sometimes ) Faiths,Data,Signal,Taxonomy of Knowledge,Facts: declarative knowledge thief(john), likes(john, wine) Rules: procedural knowledge may_steal(X, Y) if thief(X) and likes(X, Y) Control Strategy: meta, super knowledge reasoning strategy note form search strategy,Attributes of Knowledge,Range ：Special GeneralIntend ：Expository InstructionalCertainty ：Certain UncertainContain/Conflict ：Contain Conflict（in faith）,Knowledge Representation,Knowledge representation is an issue that arises in both cognitive science and AI. In cognitive science it is concerned with how people store and process information. In AI, the primary aim is to store knowledge so that programs can process it and achieve the verisimilitude of human intelligence. AI researchers have borrowed representation theories from cognitive science.,Some issues in KR,How do people represent knowledge? What is the nature of knowledge and how do we represent it? Should a representation scheme deal with a particular domain or should it be general purpose? How expressive is a representation scheme? Should the scheme be declarative or procedural?,Methodology of KR,Logic Production System Semantic Net Frame Script,Propositional Logic,Propositional logic uses true statements to form or prove other true statements. Representation (syntax): How to represent a proposition. Reasoning (algorithm): How to create or prove new propositions. Representation of propositional logic A propositional symbol and connectives (!, *, +, =, ) Example: C = “Its cold outside” ; C is a proposition O = “Its October” ; O is a proposition If O then C ;if its October then its cold outside,Predicate Logic,Same connectives as propositional logic Propositions have structure: Predicate/Function + arguments. R, 2 ; Terms. Terms are not individuals, not propositions Red(R), (Red R) ; A proposition, written in two ways (southOf UnicornCafe UniHall) ;a proposition (+ 2 2) ; Term, since the function + ranges over numbers Quantifiers enable general axioms to be written (forall ?x (iff (Triangle ?x) (and (polygon ?x) (numberOfSides ?x 3),Easy to inference,Logic as a KR language,advantages With a semantics Expressiveness Disadvantages Inefficient Undecidability Unable to express procedural knowledge Unable to do default reasoning No abduction,Production System (1),Production rules are one of the most popular and widely used knowledge representation languages Production rule system consists of three components working memory contains the information that the system has gained about the problem thus far. rule base contains information that applies to all the problems that the system may be asked to solve. interpreter solves the control problem, i.e., decide which rule to execute on each selection-execute cycle. Used both for KR and Problem solving system,Production System (2),Advantages: Naturalness of expression Modularity Restricted syntax Ability to Represent Uncertain Knowledge Disadvantages Inefficient Less expressive,Semantic Nets,Intuition base: An important feature of human memory is the high number of connections or associations between the different pieces of information contained in it. There are two types of primitive Nodes correspond to objects, or classes of objects, in the world Links are unidirectional connections between nodes and correspond to relationships between these objects,Semantic Nets,Major problem with semantic nets is that although the name of this knowledge representation language is semantic nets, there is not, ironically, clear semantics of the various network representations. For the above example, it can be interpreted as the representation of a specific bird named Tweety, or it can be interpreted as a representation of some relationship between Tweety, birds and animals.,Common used links,IS-APART-OFMODIFILES: on, down, up, bottom, moveto,Link types are set up for specific domain knowledge,Examples of Semantic Net (1),Represent a table,Analysis of Semantic Net,For a particular Domain, you make up a set of link-types create a set of nodes connect them together ascribe meaning Write Programs to manipulate the knowledge Lisp CL,Examples of Semantic Net (2),My car is tan and Johns car is green,Inference in a Semantic Net (1),Inheritance the is-a and instance-of representation provide a mechanism to implement this. Inheritance also provides a means of dealing with default reasoning,A,C,A,B,C,IS-A,IS-A,IS-A,clyde,bird,bird,fly,clyde,fly,IS-A,can,can,