Propositional Logic Arguments (5A) Young W. Lim 10/11/16

Transcription

1 Propositional Logic (5A) Young W. Lim

2 Copyright (c) 2016 Young W. Lim. Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled "GNU Free Documentation License". Please send corrections (or suggestions) to youngwlim@hotmail.com. This document was produced by using LibreOffice

3 Based on Contemporary Artificial Intelligence, R.E. Neapolitan & X. Jiang Logic and Its Applications, Burkey & Foxley 3

4 An argument consists of a set of propositions : The premises propositions The conclusion proposition List of premises followed by the conclusion A 1 A 2 A n B 4

5 Entail The premises is said to entail the conclusion If in every model in which all the premises are true, the conclusion is also true List of premises followed by the conclusion A 1 A 2 A n B whenever all the premises are true the conclusion must be true for the entailment 5

6 A Model A mode or possible world: Every atomic proposition is assigned a value T or F The set of all these assignments constitutes A model or a possible world All possible worlds (assignments) are permissiable 6

7 Entailment Notation Suppose we have an argument whose premises are A 1, A 2,, A n whose conclusion is B Then A 1, A 2,, A n B if and only if A 1 A 2 A n B (logical implication) logical implication: if A 1 A 2 A n B is tautology (always true) The premises is said to entail the conclusion If in every model in which all the premises are true, the conclusion is also true 7

8 Entailment and Logical Implication A 1, A 2,, A n B A 1 A 2 A n B A 1 A 2 A n B is a tautology (logical implication) If all the premises are true, then the conclusion must be true T T T T T T T F F X X X 8

9 Sound Argument and Fallacy A sound argument A 1, A 2,, A n B If the premises entails the conclusion A fallacy A 1, A 2,, A n B If the premises does not entail the conclusion 9

10 Entailment Examples A, B A A, B A A B A A, (A B) B A, (A B) B A (A B) B 10

11 Entailment Examples and Truth Tables A B A B A B A T T T T T F F T F T F T F F F T A B A The premises is said to entail the conclusion If in every model in which all the premises are true, the conclusion is also true any of the premises are false, still premises conclusion is true (F T and F F always T) Tautology A B A B A (A B) A (A B) B T T T T T T F F F T F T T F T F F T F T A (A B) B 11

12 Deduction System Propositional logic Given propositions (statements) : T or F Deductive inference of T or F of other propositions Deductive Inference A process by which the truth of the conclusion is shown to necessarily follow from the truth of the premises 12

13 Deduction System Deduction System : a set of inference rules Inference rules are used to reason deductively Sound Deduction System : if it derives only sound arguments Each of the inference rules is sound Complete Deduction System : It can drive every sound argument Must contain deduction theorem rule 13

14 Inference Rules Combination Rule A, B A B Simplification Rule A B A Addition Rule A A B Modus Pones A, A B B Modus Tolens B, A B A Hypothetical Syllogism A B, B C A C Disjunctive Syllogism A B, A B Rule of Cases A B, A B B Equivalence Elimination A B A B Equivalence Introduction A B, B A A B Inconsistency Rule A, A B AND Commutivity Rule A B B A OR Commutivity Rule A B B A Deduction Theorem If A 1, A 2,, A n,b C then A 1, A 2,, A n, B C 14

15 Deduction Theorem A 1, A 2,, A n, B C A 1, A 2,, A n, B C The premises is said to entail the conclusion If in every model in which all the premises are true, the conclusion is also true A 1 A 2 A n B C A 1 A 2 A n B C A 1, A 2,, A n B if and only if A 1 A 2 A n B (A 1 A 2 A n B is a tautology) A,B C B C A B C B C If A is T, then B C is always T (for the tautology) Even if A,B is T, then B C is always T And if A,B is T, then B is T By modus ponens in the RHS, A,B is T, then C is true 15

16 Deduction Theorem Δ,A B iff Δ A B where Δ : a set of formulas, if the formula B is deducible from a set Δ of assumptions, together with the assumption A, then the formula A B is deducible from Δ alone. Δ,A B Δ A B Conversely, if we can deduce A B from Δ, and if in addition we assume A, then B can be deduced. Δ A B Δ,A B 16

17 Deduction Theorem The deduction theorem conforms with our intuitive understanding of how mathematical proofs work: if we want to prove the statement A implies B, then by assuming A, if we can prove B, we have established A implies B. 17

18 Deduction Theorem The converse statement of the deduction theorem turns out to be a trivial consequence of modus ponens: A, A B B if Δ A B, then certainly Δ,A A B Since Δ,A A, we get, via modus ponens, Δ,A B as a result. Δ,A A B Δ,A A Δ,A B 18

19 Deduction Theorem Deduction theorem is needed to derive arguments that has no premises An argument without premises is simply a tautology A A no premises appear before the symbol an argument without premises Tautology if it is sound 19

20 Argument Example 1. Q 2. P Q 3. P R S 4. R T 5. U 6. U T 7. P Q, P Q P 8. R S P R S, P R S 9. T U, U T T 10. R T, R T R 11. S R S, R S 20

21 Argument without premises A A A assume A X X Y A A A A A A A A A discharge A A assume A A A X X Y A A X Y Y X A A A A A A A A A A A discharge A X Y, X Y Y A A A, A A A A A A A 21

22 Argument without premises A A assume A A A A A A A A A A A A discharge A A A assume A A A A A A A A A A A A A A A A A discharge A A A A A 22

23 To prove a sound argument Prove using truth tables Whether an argument is sound or fallacy 1. time complexity (2^n) 2. not the way which humans do Prove using inference rules To reason deductively 23

24 Double Turnstile 1. semantic consequence: a set of sentences on the left a single sentence on the right to denote that if every sentence on the left is true, the sentence on the right must be true, e.g. Γ φ. This usage is closely related to the single-barred turnstile symbol which denotes syntactic consequence. 2. satisfaction: a model (or truth-structure) on the left a set of sentences on the right to denote that the structure is a model for (or satisfies) the set of sentences, e.g. A Γ. 3. a tautology: φ to say that the expression φ is a semantic consequence of the empty set. 24

25 Syntactic Consequences A formula A is a syntactic consequence within some formal system FS of a set Γ of formulas if there is a formal proof in FS of A from the set Γ. Γ FS A Syntactic consequence does not depend on any interpretation of the formal system. A formal proof or derivation is a finite sequence of sentences (called wwf), each of which is an axiom, an assumption, or follows from the preceding sentences in the sequence by a rule of inference. The last sentence in the sequence is a theorem of a formal system. 25

26 Semantic Consequences A formula A is a semantic consequence within some formal system FS of a set of statements Γ Γ FS A if and only if there is no model I in which all members of Γ are true and A is false. the set of the interpretations that make all members of Γ true is a subset of the set of the interpretations that make A true. 26

27 Logical Consequences (1) A S B there is a derivation, in the proof-system S, from the premise A to the conclusion B. [If context fixes the relevant system S, we suppress the subscript.] A L B on every possible interpretation of the non-logical vocabulary of language L, if A comes out true, so does B. [If context fixes the relevant language L we suppress the subscript.] A B on the truth-functional interpretation, if the atomic wff A happens to be false and the atomic wff B happens to the false too, then A B evaluates as true. But we don't have A B (q isn't true on every valuation which makes p true). A B A B True True True True False False False True True False False True 27

28 Semantic Consequences (2) Syntactic consequence Γ φ sentence φ is provable from the set of assumptions Γ. Semantic consequence Γ φ sentence φ is true in all models of Γ. Soundness. If [Γ φ] then [Γ φ]. Completeness. If [Γ φ] then [Γ φ]. The propositional logic has a proof system (propositional calculus) Syntactic consequences a semantics (truth-tables) Semantic consequences 28

29 Semantic Consequences (3) A, A B B if we take the assumptions A and A B as given, by modus ponens we can deduce B. A, A B B in any model for which it is the case that A is true and also A B is true, then, in that model, B is also true. talks about the propostions themselves as syntactic objects, talks about what the propositions mean i.e. semantics. 29

30 Logical Equivalences,,,, 30

31 References [1] en.wikipedia.org [2] en.wiktionary.org [3] U. Endriss, Lecture Notes : Introduction to Prolog Programming [4] Learn Prolog Now! [5] [6] [7] [8] P. Nugues,` An Intro to Lang Processing with Perl and Prolog 31