8. Perhaps the definition may enunciate as a differentia what is really a subordinate species; or what is really the genus itself under another name; or what is not Quale, but Quid; or what belongs to the definiend as an accident only. Each of these is a locus for arguments against the definition.[292]
[292] Topica, VI. vi. p. 144, a. 5-27.
9. Perhaps also, in the definition given, the differentia or the species may be found predicable of the entire genus; or the genus may be found predicable of the differentia itself, and not of objects under it; or the species (sometimes even one of its sub-species) may be found predicable of the differentia; or perhaps the differentia may not be a prius as regards the species (which it ought to be, while it is a posterius as regards the genus). Arguments against the definition may be drawn from any one of these loci.[293]
[293] Ibid. a. 28-b. 11.
10. Recollect that the same differentia cannot belong to two distinct genera neither of which comprehends the other, unless both are comprehended under some higher genus. Examine whether this is observed in the definition tendered to you.[294]
[294] Ibid. b. 12.
11. No genuine differentia can be derived either from the Category Ubi or from the Category Passio; for neither of them furnishes characteristics essential to the subject. All Passio when intensified to a certain degree destroys the essence of the subject and removes it from its own appropriate species; but the differentia is inseparable from its subject; accordingly, nothing by virtue of which the subject is called ἀλλοῖον can be a true differentia. If the definition sins against this rule, it will be open to question.[295]
[295] Ibid. b. 31-p. 145, a. 12: ὁρᾶν δὲ καὶ εἰ τὸ ἔν τινι διαφορὰν ἀποδέδωκεν οὐσίας· οὐ δοκεῖ γὰρ διαφέρειν οὐσία οὐσίας τῷ που εἶναι. — πάλιν εἰ τὸ πάθος διαφορὰν ἀποδέδωκεν. — ἁπλῶς δ’ εἰπεῖν, καθ’ ὅσα ἀλλοιοῦται τὸ ἔχον, οὐδὲν τούτων διαφορὰ ἐκείνου· — ἁπλῶς γὰρ οὐκ ἀλλοιούμεθα κατὰ τὰς διαφοράς.
12. If the subject be relative, its true differentia ought to be relative also; thus, science or cognition is a relatum, and accordingly its three differentiæ — theoretical, practical, constructive — are all relata also.[296] The definition must conform to this; and it must also, in cases where the relative subject has more than one correlate, declare that correlate which is the ordinary and natural one, not any other which is rare and realized only on occasion.[297] You must watch to see whether this condition is observed; and also whether the correlative enunciated in the definition is the one strictly proximate. Thus, if the definition given of prudence be, It is an excellence of man or an excellence of the soul, this will not be a good definition. It ought to be — an excellence of the rational department of the soul; for it is through and by reason of this department that both man and soul are denominated prudent.[298]
[296] Ibid. a. 13.