Consider the argument:

If p, then q.	If God had wanted us to fly, He would    have given us wings.
Not-q.	He has not given us wings.
Therefore, not-p.	Therefore, He did not want us to fly.

If we assume that q is false, we can infer that p is false--for if p were not false (i.e., if it were true), then q could not have been false either.

In this case, we denied the consequent, and that allowed us to deny the antecedent. Once again, the conclusion is a categorical proposition.