L'arte di ottenere ragione/Stratagemma V/en

Stratagem V

To prove the truth of a proposition, you may also employ previous propositions that are not true, should your opponent refuse to admit the true ones, either because he fails to perceive their truth, or because he sees that the thesis immediately follows from them. In that case the plan is to take propositions which are false in themselves but true for your opponent, and argue from the way in which he thinks, that is to say, ex concessis. For a true conclusion may follow from false premisses, but not vice versâ. In the same fashion your opponent's false propositions may be refuted by other false propositions, which he, however takes to be true; for it is with him that you have to do, and you must use the thoughts that he uses. For instance, if he is a member of some sect to which you do not belong, you may employ the declared opinions of this sect against him, as principles.11

11.) Aristotle, Topica, bk. viii., chap. 2.