Quintilian, Institutio Oratoria, Book 12
Harold Edgeworth Butler, Ed.

[35] For the Academicians, although they will argue on either side of a question, do not thereby commit themselves to taking one of these two views as their guide in life to the exclusion of the other, while the famous Carneades, who is said to have spoken at Rome in the presence of Cato the Censor, and to have argued against justice with no less vigour than he had argued for justice on the preceding day, was not himself an unjust man. But the nature of virtue is revealed by vice, its opposite, justice becomes yet more manifest from the contemplation of injustice, and there are many other things that are proved by their contraries. Consequently the schemes of his adversaries should be no less well known to the orator than those of the enemy to a commander in the field.