[32] Ibid. VIII. i. p. 155, b. 10: πρὸς ἕτερον γὰρ πᾶν τὸ τοιοῦτον, τῷ δὲ φιλοσόφῳ καὶ ζητοῦντι καθ’ ἑαυτὸν οὐδὲν μέλει, ἐὰν ἀληθῆ μὲν ᾖ καὶ γνώριμα δι’ ὧν ὁ συλλογισμός, μὴ θῇ δ’ αὐτὰ ὁ ἀποκρινόμενος, &c.
[33] Ibid. i. p. 156, b. 20: χρήσιμον δὲ καὶ τὸ ἐπιλέγειν ὅτι σύνηθες καὶ λεγόμενον τὸ τοιοῦτον· ὀκνοῦσι γὰρ κινεῖν τὸ εἰωθός, ἔνστασιν μὴ ἔχοντες.
Moreover, the form of the interrogation admissible in dialectic debate is peculiar. The respondent is not bound to furnish any information in his answer: he is bound only to admit, or to deny, a proposition tendered to him. You must not ask him, What is the genus of man? You must yourself declare the genus, and ask whether he admits it, in one or other of the two following forms — (1) Is animal the genus of man? (2) Is animal the genus of man, or not? to which the response is an admission or a denial.[34]
[34] Ibid. I. iv. p. 101, b. 30. The first of these two forms Aristotle calls a πρότασις, the second he calls a πρόβλημα. But this distinction between these two words is not steadily adhered to: it is differently declared in Topic. I. x., xi. p. 104, as Alexander has remarked in the Scholia, p. 258, b. 4, Brand. Compare also De Interpretat. p. 20, b. 26; and Topic. VIII. ii. p. 158, a. 14: οὐ δοκεῖ δὲ πᾶν τὸ καθόλου διαλεκτικὴ πρότασις εἶναι, οἷον τί ἐστιν ἄνθρωπος, ἢ ποσαχῶς λέγεται τἀγαθόν; ἔστι γὰρ πρότασις διαλεκτικὴ πρὸς ἣν ἔστιν ἀποκρίνασθαι ναὶ ἢ οὔ· πρὸς δὲ τὰς εἰρημένας οὐκ ἔστιν. διὸ οὐ διαλεκτικά ἐστι τὰ τοιαῦτα τῶν ἐρωτημάτων, ἂν μὴ αὐτὸς διορίσας ἢ διελόμενος εἴπῃ.
Dialectic procedure, both of the assailant and of the defendant, has to do with propositions and problems; accordingly, Aristotle introduces a general distribution of propositions under four heads. The predicate must either be Genus, or Proprium, or Accident, of its subject. But the Proprium divides itself again into two. It always reciprocates with, or is co-extensive with, its subject; but sometimes it declares the essence of the subject, sometimes it does not. When it declares the essence of the subject, Aristotle calls it the Definition; when it does not declare the essence of the subject, although reciprocating therewith, he reserves for it the title of Proprium. Every proposition, and every problem, the entire material of Dialectic, will declare one of these four — Proprium, Definition, Genus, or Accident.[35] The Differentia, as being attached to the Genus, is ranked along with the Genus.[36]
[35] Topic. I. iv. p. 101, b. 17-36.
[36] Ibid. b. 18: τὴν διαφορὰν ὡς οὖσαν γενικὴν ὁμοῦ τῷ γένει τακτέον.
The above four general heads include all the Predicables, which were distributed by subsequent logicians (from whom Porphyry borrowed) into five heads instead of four — Genus, Species, Differentia, Proprium, Accident; the Differentia being ranked as a separate item in the quintuple distribution, and the Species substituted in place of the Definition. It is under this quadruple classification that Aristotle intends to consider propositions and problems as matters for dialectic procedure: he will give argumentative suggestions applicable to each of the four successively. It might be practicable (he thinks) to range all the four under the single head of Definition; since arguments impugning Genus, Proprium, and Accident, are all of them good also against Definition. But such a simplification would be perplexing and unmanageable in regard to dialectic procedure.[37]
[37] Topic. I. vi. p. 102, b. 27-38. ἀλλ’ οὐ διὰ τοῦτο μίαν ἐπὶ πάντων καθόλου μέθοδον ζητητέον· οὔτε γὰρ ῥᾴδιον εὑρεῖν τοῦτ’ ἐστίν, εἴ θ’ εὑρεθείη, παντελῶς ἀσαφὴς καὶ δύσχρηστος ἂν εἴη πρὸς τὴν προκειμένην πραγματείαν.
That the quadruple classification is exhaustive, and that every proposition or problem falls under one or other of the four heads, may be shown in two ways. First, by Induction: survey and analyse as many propositions as you will, all without exception will be found to belong to one of the four.[38] Secondly, by the following Deductive proof:— In every proposition the predicate is either co-extensive and reciprocating with the subject, or it is not. If it does reciprocate, it either declares the essence of the subject, or it does not: if the former, it is the Definition; if the latter, it is a Proprium. But, supposing the predicate not to reciprocate with the subject, it will either declare something contained in the Definition, or it will not. If it does contain a part of the Definition, that part must be either a Genus or a Differentia, since these are the constituents of the Definition. If it does not contain any such part, it must be an Accident.[39] Hence it appears that every proposition must belong to one or other of the four, and that the classification is exhaustive.