Mr. Poste observes upon this quadruple classification by Aristotle (p. 96):— “The two last of these are problems of Inductive, but first principles of Deductive, Science; the one being the hypothesis, the other the definition. The attribute as well as the subject must be defined (I. x.), so that to a certain degree the second problem also is assumed among the principles of Demonstration.�
Comparing together these four Quæsita, it will appear that in the first and third (Quod and An), we seek to find out whether there is or is not any middle term. In the second and fourth (Cur and Quid), we already know or assume that there is a middle term; and we try to ascertain what that middle term is.[3] The enquiry Cur, is in the main analogous to the enquiry Quid; in both cases, we aim at ascertaining what the cause or middle term is. But, in the enquiry Cur, what we discover is perhaps some independent fact or event, which is the cause of the event quæsitum; while, in the enquiry Quid, what we seek is the real essence or definition of the substance — the fundamental, generating, immanent cause of its concomitant attributes. Sometimes, however, the Quid and the Cur are only different ways of stating the same thing. E.g., Quid est eclipsis lunæ? Answer: The essence of an eclipse is a privation of light from the moon, through intervention of the earth between her and the sun. Cur locum habet eclipsis lunæ? Answer: Because the light of the sun is prevented from reaching the moon by intervention of the earth. Here it is manifest that the answers to the enquiries Quid and Cur are really and in substance the same fact, only stated in different phrases.[4]
[3] Analyt. Post. II. i. p. 889, b. 37-p. 90, a. 7. συμβαίνει ἄρα ἐν ἁπάσαις ταῖς ζητήσεσι ζητεῖν ἢ εἰ ἔστι μέσον, ἢ τί ἐστι τὸ μέσον· τὸ μὲν γὰρ αἴτιον τὸ μέσον, ἐν ἅπασι δὲ τοῦτο ζητεῖται. Compare Schol. p. 241, b. 10, Br.
[4] Analyt. Post. II. ii. p. 90, a. 14-23, 31: τὸ τί ἐστιν εἰδέναι ταὐτό ἐστι καὶ διὰ τί ἐστιν.
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]
[5] Ibid. a. 24-30. ἐκ γὰρ τοῦ αἰσθέσθαι καὶ τὸ καθόλου ἐγένετο ἂν ἡμῖν εἰδέναι· ἡ μὲν γὰρ αἴσθησις ὅτι νῦν ἀντιφράττει· καὶ γὰρ δῆλον ὅτι νῦν ἐκλείπει· ἐκ δὲ τούτου τὸ καθόλου ἂν ἐγένετο.
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.
[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]
[7] Ibid. iii. p. 90, a. 37: τί ἐστιν ὁρισμός, καὶ τίνων, εἴπωμεν, διαπορήσαντες πρῶτον περὶ αὐτῶν.