There is a locus of More and Less, which may be made available in various ways. Thus, if the genus predicated admits of being graduated as more or less, while the species of which it is predicated does not admit of such graduation, you may question the applicability of the genus to the species.[196] You may raise the question also, if there be any thing else which looks equally like the true genus, or more like it than the genus predicated by the thesis. This will happen often, when the essence of the species includes several distinct elements; e.g., in the essence of anger, there is included both pain (an emotional element), and the supposition or belief of being undervalued (an intellectual element); hence, if the thesis ranks anger under the genus pain, you may object that it equally belongs to the genus supposition[197] This locus is useful for raising a negative question, but will serve little for establishing an affirmative. Towards the affirmative, you will find advantage in examining the subject (species) respecting which the thesis predicates a given genus; for, if it can be shown that this supposed species is no real species but a genus, the genus predicated thereof will be à fortiori a genus.[198]
[196] Ibid. b. 18-25: ἔτι ἐκ τοῦ μᾶλλον καὶ ἧττον, ἀνασκευάζοντι μέν, εἰ τὸ γένος δέχεται τὸ μᾶλλον, τὸ δ’ εἶδος μὴ δέχεται μήτ’ αὐτὸ μήτε τὸ κατ’ ἐκεῖνο λεγόμενον.
[197] Ibid. b. 26-37: χρήσιμος δ’ ὁ τόπος ἐπὶ τῶν τοιούτων μάλιστα ἐφ’ ὧν πλείω φαίνεται τοῦ εἴδους ἐν τῷ τί ἐστι κατηγορούμενα, καὶ μὴ διώρισται, μήδ’ ἔχομεν εἰπεῖν ποῖον αὐτῶν γένος, &c.
[198] Ibid. b. 38-p. 128, a. 12.
Some think (says Aristotle)[199] that Differentia as well as Genus is predicated essentially respecting the Species. Accordingly, Genus must be discriminated from Differentia. For such discrimination the following characteristics are pointed out:— 1. Genus has greater extent in predication than Differentia. 2. In replying to the enquiry, Quid est? it is more suitable and significant to declare the Genus than the Differentia. 3. Differentia declares a quality of Genus, and therefore presupposes Genus as already known; but Genus does not in like manner presuppose Differentia. If you wish to show that belief is the genus to which cognition belongs, you must examine whether the cognoscens believes quâ cognoscens. If he does so, your point is made out.[200]
[199] Ibid. a. 20, seq.: ἐπεὶ δὲ δοκεῖ τισὶ καὶ ἡ διαφορὰ ἐν τῷ τί ἐστι τῶν εἰδῶν κατηγορεῖσθαι, χωριστέον τὸ γένος ἀπὸ τῆς διαφορᾶς, &c.
[200] Topic. IV. vi. p. 128, a. 35. If you are trying to show τὴν ἐπιστήμην ὅπερ πίστιν, you must examine εἰ ὁ ἐπιστάμενος ᾗ ἐπίσταται πιστεύει· δῆλον γὰρ ὅτι ἡ ἐπιστήμη πίστις ἄν τις εἴη.
Wherever a predicate is universally true of its subject, while the proposition is not true if simply converted (i.e., wherever the predicate is of larger extension than the subject), there is difficulty in distinguishing it from a genus. Accordingly, when you are respondent, maintaining the affirmative side, you will use such predicate as if it were a genus; but, when you are assailant, you will not allow the respondent to do so. You may quote against him the instance of Non-Ens; which is predicable of every thing generated, but which is not a genus, since it has no species under it.[201]
[201] Ibid. a. 38-b. 9.