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