4 Invalid Argument Forms

4 Invalid Argument Forms - Todays Topics(11/16 I II a...

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3/4/11 Today’s Topics (11/16) I. Review II. Two Invalid Argument Forms a. Denying the Antecedent b. Affirming the Consequent
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3/4/11 I. Review Modus Ponens p ⊃ q p Modus Tollens p ⊃ q ~q These are both valid argument forms. Any argument that is an instance of either of these forms will thus be valid as well.
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3/4/11 II. Two Invalid Argument Forms “Denying the Antecedent” names a particular argument form: p ⊃ q ~p In arguments of this form, one of the premises is a conditional. The other premise denies the antecedent. The conclusion denies the consequent.
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3/4/11 Proof that Denying the Antecedent is an Invalid p q p ⊃ q ~p ~q T T T F F T F F F T F T T T F F F T T T OK INVALID The table shows us that when ‘p ⊃ q’ and ‘~p’ are both true, ‘~q’ can be either true or false. Since ‘~q’ can be false under such conditions, the argument form is invalid. Notice: If ‘~q’ can be either true or false when ‘p ⊃ q’ and ‘~p’ are both true, ‘q’ can also be true or false when ‘p ⊃ q’ and ‘~p’ are both true.
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3/4/11 Implication Virtually every argument that is an instance of Denying the Since the following arguments are all instances of Denying the Antecedent, they are all invalid. 1. If James drove drunk, he broke the law. 2. James did not drive drunk. ∴ 3. James did not break the law.
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4 Invalid Argument Forms - Todays Topics(11/16 I II a...

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