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.[1]
[Footnote 1: Aristotle, _Topica_ bk. viii., chap. 2.]