That the _quæsitum_ in all these researches is a middle term or medium, is plain from those cases wherein the medium is perceivable by sense; for then we neither require nor enter upon research. For example, if we were upon the moon, we should see the earth coming between us and the sun, now and in each particular case of eclipse. Accordingly, after many such observations, we should affirm the universal proposition, that such intervention of the earth was the cause of eclipses; the universal becoming known to us through induction of particular cases.[5] The middle term, the Cause, the _Quid_, and the _Cur_, are thus all the same enquiry, in substance; though sometimes such _quæsitum_ is the quiddity or essential nature of the thing itself (as the essence of a triangle is the cause or ground of its having its three angles equal to two right angles, as well as of its other properties), sometimes it is an extraneous fact.[6] [Footnote 5: Ibid. a. 24-30. [Greek: e)k ga\r tou= ai)sthe/sthai kai\ to\ katho/lou e)ge/neto a)\n ê(mi=n ei)de/nai; ê( me\n ga\r ai)/sthêsis o(/ti nu=n a)ntiphra/ttei; kai\ ga\r dê=lon o(/ti nu=n e)klei/pei; e)k de\ tou/tou to\ katho/lou a)\n e)ge/neto.] The purport and relation of this quadruple classification of problems is set forth still more clearly in the sixth book of the Metaphysica (Z. p. 1041) with the explanations of Bonitz, Comm. pp. 358, 359.] [Footnote 6: Analyt. Post. II. ii. p. 90, a. 31.] But how or by what process is this _quæsitum_ obtained and made clear? Is it by Demonstration or by Definition? What is Definition, and what matters admit of Definition?[7] Aristotle begins by treating the question dialectically; by setting out a series of doubts and difficulties. First, Is it possible that the same cognition, and in the same relation, can be obtained both by Definition and by Demonstration? No; it is not possible. It is plain that much that is known by Demonstration cannot be known by Definition; for we have seen that conclusions both particular and negative are established by Demonstration (in the Third and Second figures), while every Definition is universal and affirmative. But we may go farther and say, that even where a conclusion universal and affirmative is established (in the First figure) by Demonstration, that same conclusion can never be known by Definition; for if it could be known by Definition, it might have been known without Demonstration. Now we are assured, by an uncontradicted induction, that this is not the fact; for that which we know by Demonstration is either a proprium of the subject _per se_, or an accident or concomitant; but no Definition ever declares either the one or the other: it declares only the essence.[8] [Footnote 7: Ibid. iii. p. 90, a. 37: [Greek: ti/ e)stin o(rismo/s, kai\ ti/nôn, ei)/pômen, diaporê/santes prô=ton peri\ au)tô=n.]]