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Lecture 5

Lecture Five: Assessing Deductive Arguments

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York University
Modes Of Reasoning
MODR 1730
Philip Mac Ewen

Lecture Five: Assessing Deductive Arguments October 5, 2011 Identifying Deductive Arguments Deductive Arguments are one of the following: - The conclusion must be true, if the premises are true. - The conclusion cannot be true, even if the premises are true Three types - Valid - Sound/Cogent - Invalid Sound Deductive Argument - Empirically true or necessarily true - Conclusion must be true because the premises are true. Example P1. 2+2 = 1+1+1+1 P2. 4 = 1+1+1+1 P3. 1+1+1+1 = 1+1+1+1 C. 2+2 = 4 Valid Deductive Argument P1. All swans are birds P2. The Swan of Tuonela is a swan C. The Swan of Tuonela is a swan (valid deductive argument because we do not know the truth of premise 2, we must make assumptions) Necessary and Sufficient Conditions In a conditional claim, the antecedent is a sufficient condition for the consequence. However, the consequent is only a necessary, not sufficient, condition for the antecedent. Example If food if delicious, then it makes my mouth water = If the food is delicious, then that is sufficient to make my mouth water Only if my mouth waters then food is delicious = It is necessary for my mouth to water in order for food to be considered delicious Stipulative Definition: A necessary condition is a condition which must be satisfied if something else to occur while a sufficient condition is a condition which, by itself, ensures that something else will occur. Supplying Missing/Assumed Premises in Deductive Arguments Terms are subjects or predicates of claims. 1. Number each term in the given claims 2. Some terms in the argument will be r
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