Affirming the consequent

Suppose in an argument one were to affirm the "then" part of a conditional (the consequent) first, and conclude with the "if" part (the antecedent).

If P, then Q.

Q.

Therefore, P.

This argument form has the name affirming the consequent, because in arguing this way one does indeed affirm the consequent in the second premise ("Q" is the consequent of the conditional claim, "If P, then Q"). This is a logical fallacy. If we argue this way, we make a mistake. One can see this with an example:

If there is fire here, then there is oxygen here. (Since oxygen is required for fire.)

There is oxygen here.

Therefore, there is fire here.

See also: modus ponens, modus tollens, denying the antecedent.