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Phil12_S11_Confirmation_falsification_fallibility(4-7-2011)

# Phil12_S11_Confirmation_falsification_fallibility(4-7-2011)...

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Valid Arguments (cont.); Confrmation, Falsifcation, and Fallibility Phil 12: Logic and Decision Making Spring 2011 UC San Diego 4/7/2011 Thursday, April 7, 2011

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Review Conditional statements - False when antecedent true, consequent false - Otherwise true Thursday, April 7, 2011
Review Rules for translating between equivalent conditional statements: - Reverse antecedent and consequent: If A then B = If not B, then not A - Only if indicates the consequent A only if B = If A, then B - Unless = If not A unless B = If not B, then A = If not A, then B Thursday, April 7, 2011

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Review Necessary & sufFcient conditions in terms of conditionals: - antecedent sufFces for consequent If A then B = A sufFces for B - consequent is necessary for the antecedent If A then B = B is necessary for A Thursday, April 7, 2011
Clicker question Consider the statement: Unless you go to medical school, your parents won't be happy. You go to medical school is: A. a necessary condition for your parents' being happy B. a sufFcient condition for your parents' being happy C. neither necessary nor sufFcient for your parents' being happy D. both necessary and sufFcient condition for your parents' being happy 5 Thursday, April 7, 2011

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Arguments using conditionals 6 Thursday, April 7, 2011
Review Valid argument: An argument in which, if the premises are true, the conclusion must also be true Thursday, April 7, 2011

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Valid conditional arguments If A, then B If A, then B A Not B B Not A Modus ponens Modus tollens Thursday, April 7, 2011
Uses of conditional arguments in scientiFc reasoning Modus ponens is most commonly invoked to make predictions from a hypothesis If malaria is transmitted by mosquitoes and we eliminate the mosquitoes, then malaria will decline. Malaria is transmitted by mosquitoes and we are eliminating the mosquitoes. Malaria will decline. Modus tollens is most commonly invoked to conFrm or falsify a hypothesis based on the truth of falsity of a prediction Thursday, April 7, 2011

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Invalid conditional arguments If A, then B Not A Not B If we assume the premises are true, must the conclusion be true? - No! Both B and not B are compatible with the premises Denying the Antecedent INVALID Thursday, April 7, 2011
Invalid conditional arguments If A, then B B A If the premises were true, must the conclusion be true in this case? - No. Both A and Not A are compatible with these premises - There is no valid argument here either! AfFrming the Consequent INVALID Thursday, April 7, 2011

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Summary: Conditional Arguments Valid argument forms: If A, then B If A, then B A Not B B Not A Modus ponens Modus tollens Invalid argument forms: If A, then B If A, then B Not A B Not B A Denying the antecedent AfFrming the consequent Thursday, April 7, 2011
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Phil12_S11_Confirmation_falsification_fallibility(4-7-2011)...

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