Denying the Antecedent

A logical condition has two parts: the antecedent and the consequent.  The antecedent (with the prefix ante-, meaning coming before, like anteroom or antebellum), indicates the first part of the condition.  If I say, "If P then Q" or "If it is raining, then the ground is wet" the antecedent is "P" or "it is raining."  The consequent is the other part of the condition, the Q, "the ground is wet."

There are two common fallacies: affirming the consequent and denying the antecedent.  If I affirm the consequent, I am saying that the ground is wet, so therefore it's raining, ignoring the possibility of the ground being wet for other reasons, such as a busted fire hydrant.  If I deny the antecedent, I say that since it is not raining, the ground cannot be wet, also ignoring the possibility of the ground being wet for other reasons.  Just because it is not raining does not mean the ground is dry.