Consider the argument:
If p, then q.
If you play with fire, you will get burned.|
You played with fire.
Therefore, you got burned.|
Any argument of this form is valid, because it merely unfolds
what is implicit in the meaning of the hypothetical premise. That
premise says that the truth of p would be sufficient for the
truth of q. If we then assume that p is true, we may conclude
that q is true as well.