It will be his business to see that he is not thus tripped up in the syllogistic process.[33] If you ask the questions in the order above indicated, without enunciating your preliminary conclusions, he must take care not to concede the same term twice, either as predicate, or as subject, or as both; for you can arrive at no conclusion unless he grants you a middle term; and no term can be employed as middle, unless it be repeated twice. Knowing the conditions of a conclusion in each of the three figures, he will avoid making such concessions as will empower you to conclude in any one of them.[34] If the thesis which he defends is affirmative, the _elenchus_ by which you impugn it must be a negative; so that he will be careful not to concede the premisses for a negative conclusion. If his thesis be negative, your purpose will require you to meet him by an affirmative; accordingly he must avoid granting you any sufficient premisses for an affirmative conclusion. He may thus make it impossible for you to prove syllogistically the contrary or contradictory of his thesis; and it is in proving this that the _elenchus_ or refutation consists. If he will not grant you any affirmative proposition, nor any universal proposition, you know, by the rules previously laid down, that no valid syllogism can be constructed; since nothing can be inferred either from two premisses both negative, or from two premisses both particular.[35] [Footnote 33: Analyt Prior. II. xix. p. 66, a. 25-32: [Greek: pro\s de\ to\ mê\ katasullogi/zesthai paratêrête/on, o(/tan a)/neu tô=n sumperasma/tôn e)rôta=| to\n lo/gon], &c. Waitz (p. 520) explains [Greek: katasullogi/zesthai], "disputationum et interrogationum laqueis aliquem irretire." This is, I think, more correct than the distinction which M. Barthélemy St. Hilaire seeks to draw, "entre le Catasyllogisme et la Réfutation," in the valuable notes to his translation of the Analytica Priora, p. 303.] [Footnote 34: Ibid. II. xix. p. 66, a. 25-32.] [Footnote 35: Ibid. xx. p. 66, b. 4-17. The reader will observe how completely this advice given by Aristotle is shaped for the purpose of obtaining victory in the argument and how he leaves out of consideration both the truth of what the opponent asks to be conceded, and the belief entertained by the defendant. This is exactly the procedure which he himself makes a ground of contemptuous reproach against the Sophists.]