Institutio Oratoria

Quintilian

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

Both these

v4-6 p.245
instances are of such a nature that the argument is reversible. For it is a necessary consequence that those who could not be taken to the province against their will could not be retained against their will.

So too I feel clear that we should rank as consequential arguments those derived from facts which lend each other mutual support and are by some regarded as forming a separate kind of argument, which they [*]( Ar. Rhet. II. xxiii. 3. ) call ἐκ τῶν πρὸς ἄλληλα, arguments from things mutually related, while Cicero [*](de Inv. I. xxx. 46. ) styles them arguments drawn from things to which the same line of reasoning applies; take the following example [*](ib. 47. ) :

If it is honourable for the Rhodians to let out their harbour dues, it is honourable likewise for Hermocreon to take the contract,
or
What it is honourable to learn, it is also honourable to teach.
Such also is the fine sentence of Domitius Afer,

which has the same effect, though it is not identical in form:

I accused, you condemned.
Arguments which prove the same thing from opposites are also mutually consequential; for instance, we may argue that he who says that the world was created thereby implies that it is suffering decay, since this is the property of all created things.

There is another very similar form of argument, which consists in the inference of facts from their efficient causes or the reverse, a process known as argument from causes. The conclusion is sometimes necessary, sometimes generally without being necessarily true. For instance, a body casts a shadow in the light, and the shadow wherever it falls indicates the presence of a body.