The first three tricks are of a kindred character. They have this in common, that something different is attacked from that which was asserted. It would therefore be an ignoratio elenchi to allow oneself to be disposed of in such a manner.
For in all the examples that I have given, what the opponent says is true, but it stands in apparent and not in real contradiction with the thesis. All that the man whom he is attacking has to do is to deny the validity of his syllogism; to deny, namely, the conclusion which he draws, that because his proposition is true, ours is false. In this way his refutation is itself directly refuted by a denial of his conclusion, per negationem consequentiae. Another trick is to refuse to admit true premisses because of a foreseen conclusion. There are two ways of defeating it, incorporated in the next two sections.
If you want to draw a conclusion, you must not let it be foreseen, but you must get the premisses admitted one by one, unobserved, mingling them here and there in your talk; otherwise, your opponent will attempt all sorts of chicanery. Or, if it is doubtful whether your opponent will admit them, you must advance the premisses of these premisses; that is to say, you must draw up pro-syllogisms, and get the premisses of several of them admitted in no definite order. In this way you conceal your game until you have obtained all the admissions that are necessary, and so reach your goal by making a circuit. These rules are given by Aristotle in his Topica, bk. viii., c. 1. It is a trick which needs no illustration.
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 versa. 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.
[Footnote 1: Aristotle, Topica bk. viii., chap. 2.]