PHIL 450A: HANDOUT #5

Four Rules of Inference (Two Valid and Two Invalid)

I. TWO DEDUCTIVELY VALID RULES OF INFERENCE

(a) Modus ponens

(1) If p, then q

(2) p

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(3) q

(b) Modus tollens

(1) If p, then q

(2) -q

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(3) -p

II. TWO DEDUCTIVELY INVALID RULES OF INFERENCE

(a) Affirming the Consequent

(1) If p, then q

(2) q

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(3) p

(b) Denying the Antecedent

(1) If p, then q

(2) -p

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(3) -q