Institutio Oratoria

For an argument must needs tell against a speaker if it be one which his opponent can use with effect.

But, you say, it is not probable that a crime of this magnitude was designed by Marcus Cotta. Is it probable then that a crime of this magnitude was attempted by Oppius?

On the other hand it is a task for a real artist to discover inconsistencies, real or apparent, in the speech of his opponent, though such inconsistencies are sometimes evident from the bare facts, as for instance in the case of Caelius, [*](pro Cael. xiii. ) where Clodia asserts on the one hand that she lent Caelius money, which is an indication of great intimacy, and on the other hand that he got poison to murder her, which is a sign of violent hatred. Tubero similarly [*](pro Liq. iii. )