Lecture 17:Inference Michael Fourman

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CHAPTER THREE Philosophical Argument

Transcription:

Lecture 17:Inference Michael Fourman

2 Is this a valid argument? Assumptions: If the races are fixed or the gambling houses are crooked, then the tourist trade will decline. If the tourist trade declines then the police force will be happy. The police force is never happy. Conclusion: The races are not fixed 2

3 Assumptions: If the races are fixed or the gambling houses are crooked, then the tourist trade will decline. If the tourist trade declines then the police force will be happy. The police force is never happy. Conclusion: The races are not fixed. we represent the argument by a deduction composed of sound deduction rules

4 assumptions X! Y Y X modus tollendo tollens conclusion A deduction rule is sound if whenever its assumptions are true then its conclusion is true If we can deduce some conclusion from a set of assumptions, using only sound rules, and the assumptions are true then the conclusion is true; the argument is valid

A! A modus tollendo tollens A A_ modus tollendo ponens A (A ^ ) modus ponendo tollens A A! modus ponendo ponens Can we find a finite set of sound rules sufficient to give a proof for any valid argument? A set of deduction rules that is sufficient to give a proof for any valid argument is said to be complete 5

Some deduction rules Are these sound? A! A modus tollendo tollens A A_ modus tollendo ponens A (A ^ ) modus ponendo tollens A A! modus ponendo ponens A _ A modus tollendo tollens A A_ modus tollendo ponens A A _ modus ponendo tollens A A _ modus ponendo ponens these rules are all equivalent to special cases of resolution, so we should expect that the answer will be yes, but we also want to formalise more natural forms of argument 6

Some sound deduction rules A! A modus tollendo tollens A A_ modus tollendo ponens A (A ^ ) modus ponendo tollens A A! modus ponendo ponens A _ A modus tollendo tollens A A_ modus tollendo ponens A A _ modus ponendo tollens A A _ modus ponendo ponens each rule corresponds to a valid entailment A!, ` A A, A _ ` A, (A ^ ) ` A, A! ` A _, ` A A, A _ ` A, A _ ` A, A _ ` 7

8 Entailment antecedents consequent A!, ` A A, A _ ` A, (A ^ ) ` A, A! ` A _, ` A A, A _ ` A, A _ ` A, A _ ` an entailment is valid if every valuation that makes all of its antecedents true makes its consequent true

9 we can use rules with entailments to formalise and study the ways we can build deductions ` A,A`, ` Cut. A. A ).. A An inference rule is sound if whenever its assumptions are valid then its conclusion is valid

10 Another rule of inference,a` ` A! (!+ ) A. ) A. A!

11 More rules - A,X ` X (I) A ` X A ` Y A ` X ^ Y (^) A,X ` Z A,Y ` Z A,X _ Y ` Z (_) A,X ` Y A ` X! Y (!) + a double line means that the rule is sound in either direction, up as well as down going down (+) introduces the connective going up (-) eliminates the connective

A simple proof A! (! C) ` A! (! C) (I) (! ) A! (! C)A `! C A! (! C), A, ` C (! ) A! (! C), ` A! C (!+ ) A! (! C) `! (A! C) (!+ ) Since each inference rule is sound if the assumptions are valid then the conclusion is valid 12 Here, we have no assumptions so the conclusion is valid.

13 More rules A,X ` X (I) A ` X A ` Y A ` X ^ Y (^) A,X ` Z A,Y ` Z A,X _ Y ` Z (_) A,X ` Y A ` X! Y (!) Can we prove X ^ Y ` X _ Y? If each inference rule is sound, then, if we can prove some conclusion (without assumptions) then the conclusion is valid

14 More rules A,X ` X (I) A ` X A ` Y A ` X ^ Y (^) A,X ` Z A,Y ` Z A,X _ Y ` Z (_) A,X ` Y A ` X! Y (!) Can we prove X ^ Y ` X _ Y? we say a set of inference rules is complete, iff if a conclusion is valid then we can prove it (without assumptions)

15 Another Proof A ^ ` A ^ (I) A ^ ` A (^ ) A _ ` A _ (I) A ` A _ (_ ) Cut A ^ ` A _ a set of entailment rules is complete if every valid entailment has a proof can we find a complete set of sound rules?

Gentzen s Rules (I) 1924, A, `,A^ ` (^L),A`,A (I) ` A,, ` A _, (_R) 1945,A`, `,A_ ` (_L) ` A, `, ` A ^, (^R) ` ` ` a sequent, Γ Δ where Γ and Δ are finite sets of expressions is valid iff whenever every expression in Γ is true some expression in Δ is true 16 Gerhard Karl Erich Gentzen (November 24, 1909 August 4, 1945)

Gentzen s Rules (I), A, `,A^ ` (^L),A`,A (I) ` A,, ` A _, (_R),A`, `,A_ ` (_L) ` A, `, ` A ^, (^R) ` a counterexample ` to the sequent ` Γ Δ, is a valuation that makes every expression in Γ true and every expression in Δ false 17 (a sequent is valid iff it has no counterexample)

18 A, ` A, (I) A ^ ` A, (^L) A ^ ` A _ (_R)

19 A rule,a`, ` A!, (! R) A valuation is a counterexample to the top line iff it is a counterexample to the bottom line

20 Another rule ` A,,A! `, ` (! L) A valuation is a counterexample to the bottom line iff it is a counterexample to at least one of the entailments on the top line

21 a valuation is a counterexample to the conclusion, A, `,A^ ` (^L),A` iff it is a counterexample to at least one assumption,a (I) ` A,, ` A _, (_R),A`, `,A_ ` (_L) ` A, `, ` A ^, (^R) ` A,, `,A! ` (! L),A`, ` A!, (! R) ` A,, A ` ( L),A` ` A, ( R)

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