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. It is commonly referred to as Modus ponens, or "method of affirming."
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.
Sometimes P and Q entail each other, in that case we can say P if and only if Q. (Sometimes the shorthand PiffQ is used rather than writing out if and only if).
Affirming the Consequent is a non-validating form of argument in propositional logic; for instance, let "p" be false and "q" be true, then there is no inconsistency in supposing that the first, conditional premiss is true, which makes the premisses true and the conclusion false.
Denying the Antecedent, instances of Affirming the Consequent are most likely to seem valid when we assume the converse of the argument's conditional premiss.
So, in general, in an instance of the form Affirming the Consequent, if it is reasonable to consider the converse of the conditional premiss to be a suppressed premiss, then the argument is not fallacious, but a valid enthymeme.
Index A, B, C, D, E, F, G, Affirming the Antecedent: In a conditional syllogism, the second premise states that the if part of the first premise is true.
Affirming the Consequent: In a conditional syllogism, the second premise states that the then part of the first premise is true.
Deny the Antecedent: In a conditional syllogism, the second premise states that the if portion of the first premise is false.
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