Chapter 21
Genus and property have this in common, namely: that they both follow the species, that is to say, are predicated of them; that they are both predicated equally of the things of which they are predicated; and that they are predicated univocally. The differences between genus and property are: that the genus is prior by nature to the property; that the genus is predicated of several species, whereas the property is predicated of one; that the property is convertible with the species, but the genus never; and that the property exists in just one species, while the genus does not.
{Κοινωνία καὶ διαφορὰ γένους καὶ ἰδίου.} Κοινὰ γένους καὶ ἰδίου: ὅτι ἀμφότερα τοῖς εἴδεσιν ἕπονται ἤγουν κατηγοροῦνται: ὅτι ἐπίσης κατηγοροῦνται, ὧν κατηγοροῦνται ἀμφότερα: ὅτι συνωνύμως κατηγοροῦνται. Διαφορὰ δὲ γένους καὶ ἰδίου: ὅτι φύσει πρότερον τὸ γένος τοῦ ἰδίου: ὅτι τὸ γένος κατὰ πλειόνων εἰδῶν κατηγορεῖται, τὸ δὲ ἴδιον ἑνός: ὅτι τὸ μὲν ἴδιον ἀντιστρέφει πρὸς τὸ εἶδος, τὸ δὲ γένος οὐκέτι: ὅτι τὸ μὲν ἴδιον μόνῳ τῷ εἴδει ὑπάρχει, τὸ δὲ γένος οὐ μόνῳ.