Explaining affirming the consequent.

Affirming the consequent is a fallacy because it is invalid but is often mistakenly thought to be valid.

If you are asked to explain why this form is invalid then you might start with the definition of validity:

Affirming the consequent must be invalid for the definition of a valid argument is one that is structured such that if the premises are true then the conclusion must be true but in this argument it is possible for the premises to be true and the conclusion false for if it is true that p implies q, and it is true that you do have q, then contrary to this argument it is still possible not to have p for the argument only says p implies q it doesn’t say q implies p.

If you are asked to give an example of the fallacy you might say something like:

If I live in Glasgow then I live in Scotland      [the consequent is in red]
I do live in Scotland      [this line affirms the consequent]
therefore, I do live in Glasgow.          


In this example the poor reasoning is obvious because it is possible for the the two premises to be true and the conclusion false. It may be that I happen to live in Edinburgh in which case both premises would be true and the conclusion false.