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Validity Part 2 - Todays Topics(9/30 I II III Review...

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3/4/11 Today’s Topics (9/30) I. Review II. Logically True Propositions III. Validity and Logically True conclusions IV. Logically Inconsistent Propositions V. Validity and Logically Inconsistent Premises
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I. Review Ø An argument is valid if and only if it is impossible for the premises to be true and the conclusion false. Ø A test for validity: If the premises were true, would Examples of Valid Arguments Examples of Invalid Arguments 1. All dogs have 8000 legs. 2. Fido is a dog. ∴ 3. Fido has 8000 legs. 1. All dolphins are mammals. 2. Some mammals have lungs. ∴ 3. All dolphins have lungs. 1. Michelle Obama is Barack Obama’s wife. 2. Barack Obama’s wife is the first lady of the U.S. ∴ 3. Michelle Obama is the first lady of the U.S. 1. Michelle Obama eats well and exercises. 2. People who eat well and exercise tend to have healthy hearts. ∴ 3. Michelle Obama has a healthy heart.
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3/4/11 A Valid Argument Can Have: True Premises & True Conclusion 1. All men are mortal. 2. Socrates is a man. ∴ 3. Socrates is mortal. False Premises & True Conclusion 1. All bananas are vegetables. 2. All vegetables are fruit. False Premises & False Conclusion 1. All bananas are vegetables.
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