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Logic and Reasoning 1 Expert Systems Dr. Samy Abu Nasser.

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1 Logic and Reasoning 1 Expert Systems Dr. Samy Abu Nasser

2 Logic and Reasoning 2 Overview Logic and Reasoning u Motivation u Objectives u Knowledge and Reasoning u logic as prototypical reasoning system u syntax and semantics u validity and satisfiability u logic languages u Reasoning Methods u propositional and predicate calculus u inference methods u Reasoning in Knowledge- Based Systems u shallow and deep reasoning u forward and backward chaining u alternative inference methods u meta-knowledge u Important Concepts and Terms u Chapter Summary

3 Logic and Reasoning 3 Logistics u Term Project u Lab and Homework Assignments u Exams u Grading

4 Logic and Reasoning 4 Dilbert on Reasoning 1

5 Logic and Reasoning 5 Dilbert on Reasoning 2

6 Logic and Reasoning 6 Dilbert on Reasoning 3

7 Logic and Reasoning 7 Pre-Test

8 Logic and Reasoning 8 Motivation u without reasoning, knowledge-based systems would be practically worthless u derivation of new knowledge u examination of the consistency or validity of existing knowledge u reasoning in KBS can perform certain tasks better than humans u reliability, availability, speed u also some limitations v common-sense reasoning v complex inferences

9 Logic and Reasoning 9 Objectives u be familiar with the essential concepts of logic and reasoning u sentence, operators, syntax, semantics, inference methods u appreciate the importance of reasoning for knowledge-based systems u generating new knowledge u explanations u understand the main methods of reasoning used in KBS u shallow and deep reasoning u forward and backward chaining u evaluate reasoning methods for specific tasks and scenarios u apply reasoning methods to simple problems

10 Logic and Reasoning 10 Chapter Introduction u Review of relevant concepts u Overview new topics u Terminology

11 Logic and Reasoning 11 Knowledge Representation Languages u syntax u sentences of the language that are built according to the syntactic rules u some sentences may be nonsensical, but syntactically correct u semantics u refers to the facts about the world for a specific sentence u interprets the sentence in the context of the world u provides meaning for sentences u languages with precisely defined syntax and semantics can be called logics

12 Logic and Reasoning 12 Sentences and the Real World u syntax u describes the principles for constructing and combining sentences v e.g. BNF grammar for admissible sentences v inference rules to derive new sentences from existing ones u semantics u establishes the relationship between a sentence and the aspects of the real world it describes u can be checked directly by comparing sentences with the corresponding objects in the real world v not always feasible or practical u complex sentences can be checked by examining their individual parts SentencesSentence

13 Logic and Reasoning 13 Diagram: Sentences and the Real World Model Sentences Sentence Follows Entails Derives Real World Syntax Semantics Symbols Symbol Strings Symbol String Semantics Syntax

14 Logic and Reasoning 14 Introduction to Logic u expresses knowledge in a particular mathematical notation All birds have wings --> ¥x. Bird(x) -> HasWings(x) u rules of inference u guarantee that, given true facts or premises, the new facts or premises derived by applying the rules are also true All robins are birds --> ¥x Robin(x) -> Bird(x) u given these two facts, application of an inference rule gives: ¥x Robin(x) -> HasWings(x)

15 Logic and Reasoning 15 Logic and Knowledge u rules of inference act on the superficial structure or syntax of the first 2 formulas u doesn't say anything about the meaning of birds and robins u could have substituted mammals and elephants etc. u major advantages of this approach u deductions are guaranteed to be correct to an extent that other representation schemes have not yet reached u easy to automate derivation of new facts u problems u computational efficiency u uncertain, incomplete, imprecise knowledge

16 Logic and Reasoning 16 Summary of Logic Languages u propositional logic u facts u true/false/unknown u first-order logic u facts, objects, relations u true/false/unknown u temporal logic u facts, objects, relations, times u true/false/unknown u probability theory u facts u degree of belief [0..1] u fuzzy logic u degree of truth u degree of belief [0..1]

17 Logic and Reasoning 17 Propositional Logic u Syntax u Semantics u Validity and Inference u Models u Inference Rules u Complexity

18 Logic and Reasoning 18 Syntax u symbols u logical constants True, False u propositional symbols P, Q, … u logical connectives v conjunction , disjunction , v negation , v implication , equivalence  u parentheses ,  u sentences u constructed from simple sentences u conjunction, disjunction, implication, equivalence, negation

19 Logic and Reasoning 19 BNF Grammar Propositional Logic Sentence  AtomicSentence | ComplexSentence AtomicSentence  True | False | P | Q | R |... ComplexSentence  (Sentence ) | Sentence Connective Sentence |  Sentence Connective   |  |  |  ambiguities are resolved through precedence      or parentheses e.g.  P  Q  R  S is equivalent to (  P)  (Q  R))  S

20 Logic and Reasoning 20 Semantics u interpretation of the propositional symbols and constants u symbols can be any arbitrary fact v sentences consisting of only a propositional symbols are satisfiable, but not valid u the constants True and False have a fixed interpretation v True indicates that the world is as stated v False indicates that the world is not as stated u specification of the logical connectives u frequently explicitly via truth tables

21 Logic and Reasoning 21 Validity and Satisfiability u a sentence is valid or necessarily true if and only if it is true under all possible interpretations in all possible worlds u also called a tautology u since computers reason mostly at the syntactic level, valid sentences are very important v interpretations can be neglected u a sentence is satisfiable iff there is some interpretation in some world for which it is true u a sentence that is not satisfiable is unsatisfiable u also known as a contradiction

22 Logic and Reasoning 22 Truth Tables for Connectives  P True True False False P  Q False True P  Q False True P  Q True False True P  Q True False True Q False True False True P False True

23 Logic and Reasoning 23 Validity and Inference u truth tables can be used to test sentences for validity u one row for each possible combination of truth values for the symbols in the sentence u the final value must be True for every sentence

24 Logic and Reasoning 24 Propositional Calculus u properly formed statements that are either True or False u syntax u logical constants, True and False u proposition symbols such as P and Q u logical connectives: and ^, or V, equivalence, implies => and not ~ u parentheses to indicate complex sentences u sentences in this language are created through application of the following rules u True and False are each (atomic) sentences u Propositional symbols such as P or Q are each (atomic) sentences u Enclosing symbols and connective in parentheses yields (complex) sentences, e.g., (P ^ Q)

25 Logic and Reasoning 25 Complex Sentences u Combining simpler sentences with logical connectives yields complex sentences u conjunction v sentence whose main connective is and: P ^ (Q V R) u disjunction u sentence whose main connective is or: A V (P ^ Q) u implication (conditional) v sentence such as (P ^ Q) => R v the left hand side is called the premise or antecedent v the right hand side is called the conclusion or consequent v implications are also known as rules or if-then statements u equivalence (biconditional) v (P ^ Q) (Q ^ P) u negation v the only unary connective (operates only on one sentence) v e.g., ~P

26 Logic and Reasoning 26 Syntax of Propositional Logic u A BNF (Backus-Naur Form) grammar of sentences in propositional logic Sentence -> AtomicSentence | ComplexSentence AtomicSentence -> True | False | P | Q | R |... ComplexSentence -> (Sentence) | Sentence Connective Sentence | ~Sentence Connective -> ^ | V | | =>

27 Logic and Reasoning 27 Semantics u propositions can be interpreted as any facts you want u e.g., P means "robins are birds", Q means "the wumpus is dead", etc. u meaning of complex sentences is derived from the meaning of its parts u one method is to use a truth table u all are easy except P => Q v this says that if P is true, then I claim that Q is true; otherwise I make no claim; v P is true and Q is true, then P => Q is true v P is true and Q is false, then P => Q is false v P is false and Q is true, then P => Q is true v P is false and Q is false, then P => Q is true

28 Logic and Reasoning 28 Exercise Semantics and Truth Tables u Use a truth table to prove the following: u P represents the fact "Wally is in location [1, 3]”: W[1,3] u H represents the fact "Wally is in location [2, 2]”: W[2,2] u We know that Wally is either in [1,3] or [2,2]: (P V H) u We learn that Wally is not in [2,2]: ~H u Can we prove that Wally is in [1,3]: ((P V H) ^ ~H) => P v This says that if the agent has some premises, and a possible conclusion, it can determine if the conclusion is true (i.e., all the rows of the truth table are true)

29 Logic and Reasoning 29 Inference Rules u more efficient than truth tables

30 Logic and Reasoning 30 Modus Ponens u eliminates => (X => Y), X ______________ Y u If it rains, then the streets will be wet. u It is raining. u Infer the conclusion: The streets will be wet. (affirms the antecedent)

31 Logic and Reasoning 31 Modus tollens (X => Y), ~Y _______________ ¬ X u If it rains, then the streets will be wet. u The streets are not wet. u Infer the conclusion: It is not raining. u NOTE: Avoid the fallacy of affirming the consequent: u If it rains, then the streets will be wet. u The streets are wet. u cannot conclude that it is raining. u If Bacon wrote Hamlet, then Bacon was a great writer. u Bacon was a great writer. u cannot conclude that Bacon wrote Hamlet.

32 Logic and Reasoning 32 Syllogism u chain implications to deduce a conclusion) (X => Y), (Y => Z) _____________________ (X => Z)

33 Logic and Reasoning 33 More Inference Rules u and-elimination u and-introduction u or-introduction u double-negation elimination u unit resolution

34 Logic and Reasoning 34 Resolution (X v Y), (~Y v Z) _________________ (X v Z) u basis for the inference mechanism in the Prolog language and some theorem provers

35 Logic and Reasoning 35 Complexity issues u truth table enumerates 2 n rows of the table for any proof involving n symbol u it is complete u computation time is exponential in n u checking a set of sentences for satisfiability is NP-complete u but there are some circumstances where the proof only involves a small subset of the KB, so can do some of the work in polynomial time u if a KB is monotonic (i.e., even if we add new sentences to a KB, all the sentences entailed by the original KB are still entailed by the new larger KB), then you can apply an inference rule locally (i.e., don't have to go checking the entire KB)

36 Logic and Reasoning 36 Inference Methods 1 u deductionsound u conclusions must follow from their premises; prototype of logical reasoning u induction unsound u inference from specific cases (examples) to the general u abduction unsound u reasoning from a true conclusion to premises that may have caused the conclusion u resolution sound u find two clauses with complementary literals, and combine them u generate and test unsound u a tentative solution is generated and tested for validity u often used for efficiency (trial and error)

37 Logic and Reasoning 37 Inference Methods 2 u default reasoning unsound u general or common knowledge is assumed in the absence of specific knowledge u analogy unsound u a conclusion is drawn based on similarities to another situation u heuristics unsound u rules of thumb based on experience u intuition unsound u typically human reasoning method u nonmonotonic reasoning unsound u new evidence may invalidate previous knowledge u autoepistemic unsound u reasoning about your own knowledge

38 Logic and Reasoning 38 Predicate Logic u new concepts (in addition to propositional logic) u complex objects v terms u relations v predicates v quantifiers u syntax u semantics u inference rules u usage

39 Logic and Reasoning 39 Objects u distinguishable things in the real world u people, cars, computers, programs,... u frequently includes concepts u colors, stories, light, money, love,... u properties u describe specific aspects of objects v green, round, heavy, visible, u can be used to distinguish between objects

40 Logic and Reasoning 40 Relations u establish connections between objects u relations can be defined by the designer or user u neighbor, successor, next to, taller than, younger than, … u functions are a special type of relation u non-ambiguous: only one output for a given input

41 Logic and Reasoning 41 Syntax u also based on sentences, but more complex u sentences can contain terms, which represent objects  constant symbols: A, B, C, Red, Square 1,3, … u stand for unique objects ( in a specific context) u predicate symbols: Adjacent-To, Younger-Than,... u describes relations between objects u function symbols: Father-Of, Square-Position, … u the given object is related to exactly one other object

42 Logic and Reasoning 42 Semantics u provided by interpretations for the basic constructs u usually suggested by meaningful names u constants u the interpretation identifies the object in the real world u predicate symbols u the interpretation specifies the particular relation in a model u may be explicitly defined through the set of tuples of objects that satisfy the relation u function symbols u identifies the object referred to by a tuple of objects u may be defined implicitly through other functions, or explicitly through tables

43 Logic and Reasoning 43 BNF Grammar Predicate Logic Sentence  AtomicSentence | Sentence Connective Sentence | Quantifier Variable,... Sentence |  Sentence | (Sentence) AtomicSentence  Predicate(Term, …)| Term = Term Term  Function(Term, …)| Constant | Variable Connective   |  |  |  Quantifier   |  Constant  A, B, C, X 1, X 2, Jim, Jack Variable  a, b, c, x 1, x 2, counter, position Predicate  Adjacent-To, Younger-Than, Function  Father-Of, Square-Position, Sqrt, Cosine ambiguities are resolved through precedence or parentheses

44 Logic and Reasoning 44 Terms u logical expressions that specify objects u constants and variables are terms u more complex terms are constructed from function symbols and simpler terms, enclosed in parentheses u basically a complicated name of an object u semantics is constructed from the basic components, and the definition of the functions involved u either through explicit descriptions (e.g. table), or via other functions

45 Logic and Reasoning 45 Unification u an operation that tries to find consistent variable bindings (substitutions) for two terms u a substitution is the simultaneous replacement of variable instances by terms, providing a “binding” for the variable u without unification, the matching between rules would be restricted to constants u often used together with the resolution inference rule u unification itself is a very powerful and possibly complex operation v in many practical implementations, restrictions are imposed v e.g. substitutions may occur only in one direction (“matching”)

46 Logic and Reasoning 46 Atomic Sentences u state facts about objects and their relations u specified through predicates and terms u the predicate identifies the relation, the terms identify the objects that have the relation  an atomic sentence is true if the relation between the objects holds u this can be verified by looking it up in the set of tuples that define the relation

47 Logic and Reasoning 47 Complex Sentences u logical connectives can be used to build more complex sentences u semantics is specified as in propositional logic

48 Logic and Reasoning 48 Quantifiers u can be used to express properties of collections of objects u eliminates the need to explicitly enumerate all objects u predicate logic uses two quantifiers u universal quantifier  u existential quantifier 

49 Logic and Reasoning 49 Universal Quantification u states that a predicate P is holds for all objects x in the universe under discourse  x P(x) u the sentence is true if and only if all the individual sentences where the variable x is replaced by the individual objects it can stand for are true

50 Logic and Reasoning 50 Existential Quantification u states that a predicate P holds for some objects in the universe  x P(x) u the sentence is true if and only if there is at least one true individual sentence where the variable x is replaced by the individual objects it can stand for

51 Logic and Reasoning 51 Horn clauses or sentences u class of sentences for which a polynomial-time inference procedure exists u P 1  P 2 ...  P n => Q where P i and Q are non-negated atomic sentences u not every knowledge base can be written as a collection of Horn sentences u Horn clauses are essentially rules of the form u If P 1  P 2 ...  P n then Q

52 Logic and Reasoning 52 Reasoning in Knowledge-Based Systems u shallow and deep reasoning u forward and backward chaining u alternative inference methods u metaknowledge

53 Logic and Reasoning 53 Shallow and Deep Reasoning u shallow reasoning u also called experiential reasoning u aims at describing aspects of the world heuristically u short inference chains u possibly complex rules u deep reasoning u also called causal reasoning u aims at building a model of the world that behaves like the “real thing” u long inference chains u often simple rules that describe cause and effect relationships

54 Logic and Reasoning 54 Examples Shallow and Deep Reasoning u shallow reasoning u deep reasoning IF a car has a good battery good spark plugs gas good tires THEN the car can move IF the battery is good THEN there is electricity IF there is electricity AND good spark plugs THEN the spark plugs will fire IF the spark plugs fire AND there is gas THEN the engine will run IF the engine runs AND there are good tires THEN the car can move

55 Logic and Reasoning 55 Forward Chaining u given a set of basic facts, we try to derive a conclusion from these facts u example: What can we conjecture about Clyde? IF elephant(x) THEN mammal(x) IF mammal(x) THEN animal(x) elephant (Clyde) modus ponens: IF p THEN q p q unification: find compatible values for variables

56 Logic and Reasoning 56 Forward Chaining Example IF elephant(x) THEN mammal(x) IF mammal(x) THEN animal(x) elephant(Clyde) modus ponens: IF p THEN q p q elephant (Clyde) IF elephant( x ) THEN mammal( x ) unification: find compatible values for variables

57 Logic and Reasoning 57 Forward Chaining Example IF elephant(x) THEN mammal(x) IF mammal(x) THEN animal(x) elephant(Clyde) modus ponens: IF p THEN q p q elephant (Clyde) IF elephant(Clyde) THEN mammal(Clyde) unification: find compatible values for variables

58 Logic and Reasoning 58 Forward Chaining Example IF elephant(x) THEN mammal(x) IF mammal(x) THEN animal(x) elephant(Clyde) modus ponens: IF p THEN q p q elephant (Clyde) IF elephant(Clyde) THEN mammal(Clyde) IF mammal( x ) THEN animal( x ) unification: find compatible values for variables

59 Logic and Reasoning 59 Forward Chaining Example IF elephant(x) THEN mammal(x) IF mammal(x) THEN animal(x) elephant(Clyde) modus ponens: IF p THEN q p q elephant (Clyde) IF elephant(Clyde) THEN mammal(Clyde) IF mammal(Clyde) THEN animal(Clyde) unification: find compatible values for variables

60 Logic and Reasoning 60 Forward Chaining Example IF elephant(x) THEN mammal(x) IF mammal(x) THEN animal(x) elephant(Clyde) modus ponens: IF p THEN q p q elephant (Clyde) IF elephant(Clyde) THEN mammal(Clyde) IF mammal(Clyde) THEN animal(Clyde) animal( x ) unification: find compatible values for variables

61 Logic and Reasoning 61 Forward Chaining Example IF elephant(x) THEN mammal(x) IF mammal(x) THEN animal(x) elephant(Clyde) modus ponens: IF p THEN q p q elephant (Clyde) IF elephant(Clyde) THEN mammal(Clyde) IF mammal(Clyde) THEN animal(Clyde) animal(Clyde) unification: find compatible values for variables

62 Logic and Reasoning 62 Backward Chaining u try to find supportive evidence (i.e. facts) for a hypothesis u example: Is there evidence that Clyde is an animal? IF elephant(x) THEN mammal(x) IF mammal(x) THEN animal(x) elephant (Clyde) modus ponens: IF p THEN q p q unification: find compatible values for variables

63 Logic and Reasoning 63 Backward Chaining Example IF elephant(x) THEN mammal(x) IF mammal(x) THEN animal(x) elephant(Clyde) modus ponens: IF p THEN q p q IF mammal( x ) THEN animal( x ) animal(Clyde) unification: find compatible values for variables ?

64 Logic and Reasoning 64 Backward Chaining Example IF elephant(x) THEN mammal(x) IF mammal(x) THEN animal(x) elephant(Clyde) modus ponens: IF p THEN q p q IF mammal(Clyde) THEN animal(Clyde) animal(Clyde) unification: find compatible values for variables ?

65 Logic and Reasoning 65 Backward Chaining Example IF elephant(x) THEN mammal(x) IF mammal(x) THEN animal(x) elephant(Clyde) modus ponens: IF p THEN q p q IF elephant( x ) THEN mammal( x ) IF mammal(Clyde) THEN animal(Clyde) animal(Clyde) unification: find compatible values for variables ? ?

66 Logic and Reasoning 66 Backward Chaining Example IF elephant(x) THEN mammal(x) IF mammal(x) THEN animal(x) elephant(Clyde) modus ponens: IF p THEN q p q IF elephant(Clyde) THEN mammal(Clyde) IF mammal(Clyde) THEN animal(Clyde) animal(Clyde) unification: find compatible values for variables ? ?

67 Logic and Reasoning 67 Backward Chaining Example IF elephant(x) THEN mammal(x) IF mammal(x) THEN animal(x) elephant(Clyde) modus ponens: IF p THEN q p q elephant ( x ) IF elephant(Clyde) THEN mammal(Clyde) IF mammal(Clyde) THEN animal(Clyde) animal(Clyde) unification: find compatible values for variables ? ? ?

68 Logic and Reasoning 68 Backward Chaining Example IF elephant(x) THEN mammal(x) IF mammal(x) THEN animal(x) elephant(Clyde) modus ponens: IF p THEN q p q elephant (Clyde) IF elephant(Clyde) THEN mammal(Clyde) IF mammal(Clyde) THEN animal(Clyde) animal(Clyde) unification: find compatible values for variables

69 Logic and Reasoning 69 Forward vs. Backward Chaining Forward ChainingBackward Chaining planning, controldiagnosis data-drivengoal-driven (hypothesis) bottom-up reasoningtop-down reasoning find possible conclusions supported by given facts find facts that support a given hypothesis similar to breadth-first search similar to depth-first search antecedents (LHS) control evaluation consequents (RHS) control evaluation

70 Logic and Reasoning 70 Alternative Inference Methods u theorem proving u emphasis on mathematical proofs, not so much on performance and ease of use u probabilistic reasoning u integrates probabilities into the reasoning process u fuzzy reasoning u enables the use of ill-defined predicates

71 Logic and Reasoning 71 Metaknowledge u deals with “knowledge about knowledge” u e.g. reasoning about properties of knowledge representation schemes, or inference mechanisms u usually relies on higher order logic v in (first order) predicate logic, quantifiers are applied to variables v second-order predicate logic allows the use of quantifiers for function and predicate symbols v equality is an important second order axiom »two objects are equal if all their properties (predicates) are equal v may result in substantial performance problems

72 Logic and Reasoning 72 Important Concepts and Terms u and operator u atomic sentence u backward chaining u existential quantifier u expert system shell u forward chaining u higher order logic u Horn clause u inference u inference mechanism u If-Then rules u implication u knowledge u knowledge base u knowledge-based system u knowledge representation u matching u meta-knowledge u not operator u or operator u predicate logic u propositional logic u production rules u quantifier u reasoning u rule u satisfiability u semantics u sentence u symbol u syntax u term u validity u unification u universal quantifier

73 Logic and Reasoning 73 Summary Reasoning u reasoning relies on the ability to generate new knowledge from existing knowledge u implemented through inference rules v related terms: inference procedure, inference mechanism, inference engine u computer-based reasoning relies on syntactic symbol manipulation (derivation) u inference rules prescribe which combination of sentences can be used to generate new sentences u ideally, the outcome should be consistent with the meaning of the respective sentences (“sound” inference rules) u logic provides the formal foundations for many knowledge representation schemes u rules are frequently used in expert systems


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