A only if B is logically equivalent to if A， then B.

　　Affirming the Conclusion Fallacy

　　A――B

　　B

　　Therefore， A

　　Remember that an if-then statement， A――B， tells us only two things： If　A is true， then B is true as well. If B is false， then A is false as well 。 If， however， we know the conclusion is true， the if-then　statement tells us nothing about the premise. And if we know that the premise is false ， then the if-then statement tells us nothing about the conclusion.

　　Example：

　　If he is innocent， then when we hold him under water for sixty seconds he will not drown. Since he did not die when we dunked him in the water， he must　be innocent.

　　The logical structure of the argument above is most similar to which one of　the following?

　　 To insure that the remaining wetlands survive， they must be protected by　the government. This particular wetland is being neglected. Therefore， it will soon perish.

　　 There were nuts in that pie I just ate. There had to be， because when I　eat nuts I break out in hives， and I just noticed a blemish on my hand.

　　 The president will be reelected unless a third candidate enters the race A third candidate has entered the race， so the president will not be reelected.

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