# MP and MT - Todays Topics(11/9 I a Two Common Valid...

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3/4/11 Today’s Topics (11/9) I. Two Common Valid Argument Forms a. Modus Ponens b. Modus Tollens II. A Point about Validity III. Some Other Rules of Inference

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3/4/11 II. Two Common Valid Argument Forms “Modus Ponens” names a particular argument form: p ⊃ q p In arguments of this form, one of the premises is a conditional. The other premise affirms the antecedent. The conclusion affirms the consequent. “Modus Ponens” is sometimes called “ Affirming the Antecedent .”
3/4/11 Proof that Modus Ponens is a Valid Argument Form p q p ⊃ q p q T T T T T T F F T F F T T F T F F T F F OK

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3/4/11 Implication Every argument that is an instance of a valid argument form Since the following arguments are all instances of Modus Ponens, they are all valid. 1. If James drove drunk, he broke the law. 2. James drove drunk. ∴ 3. James broke the law. 1. If James did not break the law, he did not drive drunk. 2. James did not break the law. ∴ 3. James did not drive drunk. 1. If the president is a democrat and most of the senators are democrats, then the Republicans are not happy. 2. The president is a democrat and most of the senators are democrats. ∴ 3. The Republicans are not happy.
Modus Ponens as a Rule of Inference From premises of the form “If p then q” and “p” you can validly infer “q.” Suppose you believe that a. If your wife is not home, she is having an affair; and b. Your wife is not home. You can validly infer that your wife is having an affair.

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MP and MT - Todays Topics(11/9 I a Two Common Valid...

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