Essence or Substance is not in a Subject; neither First nor Second Essence. The First Essence is neither in a Subject nor predicated of a Subject; the Second Essences are not in the First, but are predicated of the First. Both the Second Essence, and the definition of the word describing it, may be predicated of the First; that is, the predication is synonymous or univocal; whereas, of that which is in a Subject, the name may often be predicated, but never the definition of the name. What is true of the Second Essence, is true also of the Differentia; that it is not in a Subject, but that it may be predicated univocally of a Subject — not only its name, but also the definition of its name.[39]
[39] Ibid. p. 3, a. 7, 21, 34. κοινὸν δὲ κατὰ πάσης οὐσίας τὸ μὴ ἐν ὑποκειμένῳ εἶναι — οὐκ ἴδιον δὲ τῆς τοῦτο οὐσίας, ἀλλὰ καὶ ἡ διαφορὰ τῶν μὴ ἐν ὑποκειμένῳ ἐστίν — ὑπάρχει δὲ ταῖς οὐσίαις καὶ ταὶς διαφοραῖς τὸ πάντα συνωνύμως ἀπ’ αὐτῶν λέγεσθαι.
All Essence or Substance seems to signify Hoc Aliquid Unum Numero. The First Essence really does so signify, but the Second Essence does not really so signify: it only seems to do so, because it is enunciated by a substantive name, like the First.[40] It signifies really Tale Aliquid, answering to the enquiry Quale Quid? for it is said not merely of one thing numerically, but of many things each numerically one. Nevertheless, a distinction must be drawn. The Second Essence does not (like the Accident, such as white) signify Tale Aliquid simply and absolutely, or that and nothing more. It signifies Talem Aliquam Essentiam; it declares what the Essence is, or marks off the characteristic feature of various First Essences, each Unum Numero. The Genus marks off a greater number of such than the Species.[41]
[40] Aristot. Categ. p. 3, b. 10-16: Πᾶσα δὲ οὐσία δοκεῖ τόδε τι σημαίνειν. ἐπὶ μὲν οὖν τῶν πρώτων οὐσιῶν ἀναμφισβήτητον καὶ ἀληθές ἐστιν ὅτι τόδε τι σημαίνει· ἄτομον γὰρ καὶ ἓν ἀριθμῷ τὸ δηλούμενόν ἐστιν· ἐπὶ δὲ τῶν δευτέρων οὐσιῶν φαίνεται μὲν ὁμοίως τῷ σχήματι τῆς προσηγορίας τόδε τι σημαίνειν, ὅταν εἴπῃ ἄνθρωπον ἢ ζῷον, οὐ μὴν ἀληθές γε, ἀλλὰ μᾶλλον ποιόν τι σημαίνει.
[41] Ibid. p. 3, b. 18-24.
Again, Essences have no contraries.[42] But this is not peculiar to Essences, for Quanta also have no contraries; there is nothing contrary to ten, or to that which is two cubits long. Nor is any one of the varieties of First Essence more or less Essence than any other variety. An individual man is as much Essence as an individual horse, neither more nor less. Nor is he at one time more a man than he was at another time; though he may become more or less white, more or less handsome.[43]
[42] Ibid. b. 24-30.
[43] Ibid. b. 34, seq.
But that which is most peculiar to Essence, is, that while remaining Unum et Idem Numero, it is capable by change in itself of receiving alternately contrary Accidents. This is true of no other Category. For example, this particular colour, being one and the same in number, will never be now black, and then white; this particular action, being one and the same in number, will not be at one time virtuous, at another time vicious. The like is true respecting all the other Categories. But one and the same man will be now white, hot, virtuous; at another time, he will be black, cold, vicious. An objector may say that this is true, not merely of Essence, but also of Discourse and of Opinion; each of which (he will urge) remains Unum Numero, but is nevertheless recipient of contrary attributes; for the proposition or assertion, Sokrates is sitting, may now be true and may presently become false. But this case is different, because there is no change in the proposition itself, but in the person or thing to which the proposition refers; while one and the same man, by new affections in himself, is now healthy, then sick; now hot, then cold.[44]
[44] Aristot. Categ. p. 4, a. 10-b. 20.