PHIL 210 Lecture Notes - Lecture 3: Modus Tollens, Modus Ponens, Critical Thinking
8 May 2020
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Modus ponens: if p then q, p, therefore, q. Modus tollens: if p then q, not q, therefore, not p. These both get their names from what the second premise does: denying the antecedent: If p then q: not p, therefore, not q, affirming the consequent: If p then q: q, therefore, p. The concepts of necessary and sufficient conditions are very important. Being over 5 feet tall is necessary for being over six feet tall. To say a is sufficient for b is to say if you have a, then you also must have b. Being over 6 feet tall is sufficient for being over 5 feet tall. (you may notice, as demonstrated in the examples, that if a is necessary for b, then b is sufficient for a. ) Many instances of communication are explicitly or implicitly arguments, even though they may not have clearly designated premises and conclusions.
