Institutio Oratoria
Quintilian
Quintilian. Institutio Oratoria, Volume 1-4. Butler, Harold Edgeworth, translator. Cambridge, Mass; London: Harvard University Press, William Heinemann Ltd., 1920-1922.
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
Some call the enthymeme a rhetorical syllogism, while others regard it as a part of the syllogism, because whereas the latter always has its premises and conclusion and effects its proof by the employment of all its parts, the ethymeme is content to let its proof be understood without explicit statement.
The following is an example of a syllogism:
Virtue is the only thing that is good, for that alone is good which no one can put to a bad use: but no one can make a bad use of virtue; virtue therefore is good.The enthymeme draws its conclusion from denial of consequents.
Virtue is a good thing because no one can put it to a bad use.On the other hand take the following syllogism.
Money is not a good thing; for that is not good which can be put to a bad use: money may be put to a bad use; therefore money is not a good thing.The enthymeme draws its conclusion from incompatibles.
Can money be a good thing when it is possible to put it to a bad use?