Institutio Oratoria

The minor premise is refuted by all the methods which we mentioned in dealing with refutation. [*](In the preceding chapter.) As to the reason it must be pointed out that it is sometimes true when the proposition to which it is attached is not true, but may on the other hand sometimes be false although the proposition is true. For example,

Virtue is a good thing

is true, but if the reason,

Because it brings us wealth,

be added, we shall have an instance of a true major premise and a false reason.

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.

But while the major premise and the reason

may both be true and the conclusion false, yet if both are false, the conclusion can never be true.