In the Second and Third figures, though not in the First, it is possible to obtain conclusions even from two premisses which are contradictory or contrary to each other; but the conclusion will, as a matter of course, be a self-contradictory one. Thus if in the Second figure you have the two premisses--All Science is good; No Science is good--you get the conclusion (in _Camestres_), No Science is Science. In opposed propositions, the same predicate must be affirmed and denied of the same subject in one of the three different forms--All and None, All and Not All, Some and None. This shows why such conclusions cannot be obtained in the First figure; for it is the characteristic of that figure that the middle term must be predicate in one premiss, and subject in the other.[17] In dialectic discussion it will hardly be possible to get contrary or contradictory premisses conceded by the adversary immediately after each other, because he will be sure to perceive the contradiction: you must mask your purpose by asking the two questions not in immediate succession, but by introducing other questions between the two, or by other indirect means as suggested in the Topica.[18] [Footnote 17: Analyt. Prior. II. xv. p. 63, b. 22-p. 64, a. 32. Aristotle here declares _Subcontraries_ (as they were later called),--Some men are wise, Some men are not wise,--to be opposed only in expression or verbally ([Greek: kata\ tê\n le/xin mo/non]).] [Footnote 18: Ibid. II. xv. p. 64, a. 33-37. See Topica, VIII. i. p. 155, a. 26; Julius Pacius, p. 372, note. In the Topica, Aristotle suggests modes of concealing the purpose of the questioner and driving the adversary to contradict himself: [Greek: e)n de\ tô=s Topikoi=s paradi/dôsi metho/dous tô=n kru/pseôn di' a(\s tou=to dothê/setai] (Schol. p. 192, a. 18, Br.). Compare also Analyt. Prior. II. xix. p. 66, a. 33.] Aristotle now passes to certain general heads of Fallacy, or general liabilities to Error, with which the syllogizing process is beset. What the reasoner undertakes is, to demonstrate the conclusion before him, and to demonstrate it in the natural and appropriate way; that is, from premisses both more evident in themselves and logically prior to the conclusion. Whenever he fails thus to demonstrate, there is error of some kind; but he may err in several ways: (1) He may produce a defective or informal syllogism; (2) His premisses may be more unknowable than his conclusion, or equally unknowable; (3) His premisses, instead of being logically prior to the conclusion, may be logically posterior to it.[19] [Footnote 19: Ibid. II. xvi. p. 64, b. 30-35: [Greek: kai\ ga\r ei) o(/lôs mê\ sullogi/zetai, kai\ ei) di' a)gnôstote/rôn ê)\ o(moi/ôs a)gnô/stôn, kai\ ei) dia\ tô=n u(ste/rôn to\ pro/teron; ê( ga\r a)po/deixis e)k pistote/rôn te kai\ prote/rôn e)stin.... ta\ _me\n di' au(tô=n pe/phuke gnôri/zesthai, ta\ de\ di' a)/llôn_.]]