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.