Chapter 20
Genus and species have this in common: that they are predicated of the essence of several things; that by nature they are prior to those things that come under them; and that each is a whole something. Distinguishing peculiarities of genus and species are as follow, namely: that the genus is more general than the species; that the species is richer in differences than the genus; that the genus is predicated of the species univocally, whereas the species is not convertible; and that neither is the genus more specific, nor the species most general, nor can that which is most specific be a genus.
{Κοινωνία καὶ διαφορὰ γένους καὶ εἴδους.} Κοινὰ γένους καὶ εἴδους: τὸ κατὰ πλειόνων ἐν τῷ τί ἐστι κατηγορεῖσθαι: τὸ φύσει πρότερα εἶναι τῶν ὑπ' αὐτά: τὸ ὅλον τι εἶναι ἀμφότερα. Ἴδια δὲ γένους καὶ εἴδους: ὅτι τὸ γένος καθολικώτερόν ἐστι τοῦ εἴδους: ὅτι τὸ εἶδος περιττεύει τοῦ γένους ταῖς διαφοραῖς: ὅτι τὸ γένος συνωνύμως κατηγορεῖται τοῦ εἴδους, τὸ δὲ εἶδος οὐκ ἀντιστρέφει: ὅτι οὔτε τὸ γένος εἰδικώτατον οὔτε τὸ εἶδος γενικώτατον οὔτε τὸ γενικώτατον ἔσται εἶδος οὔτε τὸ εἰδικώτατον ἔσται γένος.