§1.2.12 Λαμβάνουσι δʼ ἔνιοι τὸ καθʼ ἕκαστον, διαιρούμενοι τὸ γένος εἰς δύο διαφοράς.
2 Some people grasp the particular by dividing the genus into two differences.
τοῦτο δʼ ἐστὶ τῇ μὲν οὐ ῥᾴδιον, τῇ δὲ ἀδύνατον.
But this is in one way not easy, and in another way impossible.
ἐνίων γὰρ ἔσται διαφορὰ μία μόνη, τὰ δʼ ἄλλα περίεργα, οἷον ὑπόπουν, δίπουν, σχιζόπουν, ἄπουν· αὕτη γὰρ μόνη κυρία.
For in some cases there will be only one difference, while the others are superfluous, such as having feet, two-footed, cloven-footed, footless; for this one alone is decisive.
εἰ δὲ μὴ, ταὐτὸν πολλάκις ἀναγκαῖον λέγειν.
Otherwise, it is necessary to say the same thing many times.
ἔτι δὲ προσήκει μὴ διασπᾶν ἕκαστον γένος, οἷον τοὺς ὄρνιθας τοὺς μὲν ἐν τῇδε τοὺς δʼ ἐν ἄλλῃ διαιρέσει, καθάπερ ἔχουσιν αἱ γεγραμμέναι διαιρέσεις· ἐκεῖ γὰρ τοὺς μὲν μετὰ τῶν ἐνύδρων συμβαίνει διῃρῆσθαι, §1.2.22 τοὺς δʼ ἐν ἄλλῳ γένει.
Furthermore, it is appropriate not to tear apart each genus, such as birds, by putting some in this division and others in another, just as the written divisions have it; for there it happens that some are divided along with aquatic animals, 2 and others in another genus.
ταύτῃ μὲν οὖν τῇ ὁμοιότητι ὄρνις ὄνομα κεῖται, ἑτέρᾳ δʼ ἰχθύς. ἄλλαι δʼ εἰσὶν ἀνώνυμοι, σἷον τὸ ἔναιμον καὶ τὸ ἄναιμον· ἐφʼ ἑκατέρῳ γὰρ τούτων οὐ κεῖται ἓν ὄνομα.
Therefore, for this similarity the name "bird" is established, and for the other "fish." But others are nameless, such as the blooded and the bloodless; for no single name is established for each of these.
εἴπερ οὖν μηδὲν τῶν ὁμογενῶν διασπαστέον, ἡ εἰς δύο διαίρεσις μάταιος ἂν εἴη· οὕτω γὰρ διαιροῦντας ἀναγκαῖον χωρίζειν καὶ διασπᾶν· τῶν πολυπόδων γάρ ἐστι τὰ μὲν ἐν τοῖς πεζοῖς τὰ δʼ ἐν τοῖς ἐνύδροις.
If, then, none of the homogeneous things is to be torn apart, division into two would be futile; for to those who divide in this way, it is necessary to separate and tear apart; for of the many-footed animals, some are among the land animals, and others among the aquatic ones.