[248] Ibid. b. 28-p. 138, a. 3.
[249] Topica, V. viii. p. 138, a. 4-12.
[250] Ibid. p. 138, a. 13-20: κατασκευάζοντι δὲ ὁ τόπος οὗτος οὔκ ἐστι χρήσιμος· ἀδύνατον γάρ ἐστι ταὐτὸ πλειόνων ἴδιον εἶναι.
[251] Ibid. a. 21-30.
22. Locus from Equal Relation:— Arguments both negative and affirmative may in like manner be obtained by comparing different things which are (not more or less propria, but) alike or equally propria of some other subject. If A is as much a proprium of B as C is proprium of D, while yet A is not a proprium of B, you may hence infer that C is not a proprium of D. If, under this hypothesis, A is a proprium of B, you may infer affirmatively that C is a proprium of D.[252] Or, if A and C be, alike and equally, propria of the same subject B, then, if you show that A is not proprium thereof, you will infer negatively that C is not so; if you show that A is proprium of B, you will infer affirmatively that C is so likewise. Or, thirdly, if A be, alike and equally, a proprium of B and of E, then, if you can show that A is not a proprium of E, you may infer negatively that it is not a proprium of B. Here, however, the counter-inference affirmatively is not allowable; for the same proprium cannot belong as proprium to two distinct subjects, as was stated before.[253]
[252] Ibid. a. 30-b. 15.
[253] Ibid. b. 16-22.
23. Locus from Potentiality:— No potentiality whatever can belong to Non-Ens. Accordingly, if A, the proprium affirmed of a subject B, is a potentiality, this must imply some real Ens in which it inheres, and which is correlate to the subject. But, if in the specification of the proprium no allusion is made to such correlate, you will attack it as a bad proprium — as a potentiality inhering in Non-Ens or nothing. E.g., if the case be, It is a proprium of air to be respirable, you will refute this by pointing out that this is true only when there exist animals in whom the potentiality of breathing resides; that no mention is made by the respondent of this correlate or of any other correlate; in other words, that, so far as the specification is concerned, the correlate is passed over as Non-Ens or a non-entity. Therefore the proprium is not a good proprium.[254] Again, suppose the affirmation to be, It is a proprium of Ens to be capable of doing or suffering something; this will be defensible because it is only when the subject is Ens, that it is declared to have such proprium.[255]
[254] Topica, V. ix. p. 138, b. 27-37. οἷον ἐπεὶ ὁ εἴπας ἀέρος ἴδιον τὸ ἀναπνευστόν τῇ δυνάμει μὲν ἀπέδωκε τὸ ἴδιον (τὸ γὰρ τοιοῦτον ἴδιον οἷον ἀναπνεῖσθαι ἀναπνευστόν ἐστιν), ἀποδέδωκε δὲ καὶ πρὸς τὸ μὴ ὃν τὸ ἴδιον· καὶ γὰρ μὴ ὄντος ζῴου, οἷον ἀναπνεῖν πέφυκε τὸν ἀέρα, ἐνδέχεται ἀέρα εἶναι· οὐ μέντοι μὴ ὄντος ζῴου δυνατόν ἐστιν ἀναπνεῖν· ὥστ’ οὐδ’ ἀέρος ἔσται ἴδιον τὸ τοιοῦτον οἷον ἀναπνεῖσθαι, τότε ὅτε ζῴον οὐκ ἔσται τοιοῦτον οἷον ἀναπνεῖν. οὐκ ἂν οὖν εἴη ἀέρος ἴδιον τὸ ἀναπνευστόν.
Respirability (the proprium here discussed) being a relative term, Aristotle demands that the correlate thereof shall be named and included in setting out the proprium. If this be not done, a refutative argument may be drawn from such omission — that the respondent was not aware of the relativity. We may remark here that this objection is founded on a bad or incomplete specification of the proprium in question: it is not an objection against the reality of that proprium itself, if carefully described. The objection belongs to that class which Aristotle had discussed before, at the commencement of Book V.