Institutio Oratoria

With regard to the conclusion, we may either deny its truth when it infers something which does not logically result from the premises, or we may treat it as irrelevant. The truth is denied in the following case:

We are justified in killing one who lies in wait for us; for since, like an enemy, he threatens us with violence, we ought to repulse his attack as though he were an enemy: therefore Milo was justified in killing Clodius as an enemy.

The conclusion is not valid, since we have not yet proved that Clodius lay in wait for him But the conclusion that we are therefore justified in killing one who lies in wait for us is perfectly true, though irrelevant to the case, for it is not yet clear that Clodius lay in wait for Milo.