PHIL 2110 Lecture Notes - Lecture 6: Modus Tollens, Logical Biconditional, Principle Of Bivalence
4 Feb 2016
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Fallacy of affirming the consequent: if you are human you are mortal. Therefore, you are human can"t affirm the antecedent from the consequent unless it is a biconditional: w b, b, w - this is invalid, if airlines were lighter than air, they"d fly above the ground. If six turned out to be nine, i don"t mind. It is false that i don"t mind if six turned out to be nine. (s m: the rule of inference double negation (dn) is: From any statement infer the negation of its negation, and vice versa. The validity of (1): you cannot asset the truth of p and at the same time assert. The validity of (2), however, relies on the principle of bivalence: every statement is either true or false; there is no third option, such as it being undecidable whether it is true or false.
