Affirming the antecedent is a valid argument form which proceeds by affirming the truth of the first part (the "if" part, commonly called the antecedent) of a conditional, and concluding that the second part (the "then" part, commonly called the consequent) is true.
- If P, then Q.
- P.
- Therefore, Q.
- If P then Q.
- Q.
- Therefore P.
But this is a Logical fallacy called Affirming the consequent. Since P implies Q, but Q does not necessarily imply P.
You can see this if we simply substitute in actuall statements for P. and Q.
- If there is fire here, then there is oxygen here.
- There is oxygen here.
- Therefore, there is fire here.