Institutio Oratoria

In a similar way division is valuable both for proof and refutation. For proof, it is sometimes enough to establish one thing.

To be a citizen, a man must either have been born or made such.

For refutation, both points must be disproved:

he was neither born nor made a citizen.

This may be done in many ways, and constitutes a form of argument by elimination, whereby we show sometimes that the whole is false, sometimes that only that which remains alter the process of elimination is true. An example of the first of these two cases would be:

You say that you lent him money. Either you possessed it yourself, received it from another, found it or stole it. If you did not possess it, receive it from another, find or steal it, you did not lend it to him.

The residue after elimination is shown to be true as follows:

This slave whom you claim was either born in your house or bought or given you or left you by will or captured from the enemy or belongs to another.

By the elimination of the previous suppositions he is shown to belong to another. This form of argument is risky and must be employed with care; for if, in setting forth the alternatives, we chance to omit one, our whole case will fail, and our audience will be moved to laughter. It is safer to do what Cicero [*](pro Caec. xiii. 37. )

does in the pro Caecina, when he asks,

If this is not the point at issue, what is?

For thus all other points are eliminated at one swoop. Or again two contrary propositions may be advanced, either of which if established would suffice to prove the case. Take the following example from Cicero: [*](pro Cluent. xxiii. 64. )

There can be no one so hostile to Cluentius as not to grant me one thing: if it be a fact that the verdict then given was the result of bribery, the bribes must have proceeded either from Habitus or Oppianicus: if I show that they did not proceed from Habitus I prove that they proceeded from Oppianicus: if I demonstrate that they were given by Oppianicus, I clear Habitus.