Institutio Oratoria

Quintilian

Quintilian. Institutio Oratoria, Volume 1-4. Butler, Harold Edgeworth, translator. Cambridge, Mass; London: Harvard University Press, William Heinemann Ltd., 1920-1922.

On the other hand, he places the various parts in the regular order in the following instance:

How can it be unjust to kill a robber who lies in wait for his victim?
[*](ib. iv. 10. ) Next comes the reason:
What is the object of our escorts and our swords?
Last comes the conclusion resulting from the major premise and the reason:
Which we certainly should not be permitted to have, if we were absolutely forbidden to use them.
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This form of proof may be countered in three ways, that is to say it may be attacked in all its parts. For either the major premise or the minor or the conclusion or occasionally all three are refuted. The major premise is refuted in the following case:

I was justified in killing him, as he lay in wait for me.
For the very first question in the defence of Milo is
whether it is right that he who confesses that he has killed a man should look upon the light of day.

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

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

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?

The following argument is couched in syllogistic form:

If money in the form of silver coin is silver, the man who bequeathed all his silver to a legatee, includes all money in the form of coined silver: but he bequeathed all his silver: therefore he included in the bequest all money in the form of coined silver.
But for the orator it will be sufficient to say,
Since he bequeathed all his silver, he included in his bequest all his silver money.