A genus is that which is predicated of many things that also differ in their form with respect to their essence. As many things as he who defines appropriately, “What is the thing?,” sitting before him after he has been asked, such things are said to be predicated in their essence. Just as, in the case of “human,” someone appropriately responds that what lies before him is an animal. Connected with the genus as well is whether a thing is in a genus that is the same as another thing or different from another thing. For such a thing falls under a method the same as genus. For having said that “animal” is the genus of human, and likewise the genus of cow, we shall have said that they are in the same genus. But if we shall prove that it is the genus of one, but it is not the genus of the other, we shall have proved that these are not in the same genus. Topica 102a32-102b3

A genus differs in its form, in that essentially, a horse is different from a dog, yet both are nevertheless part of the same genus. Genus is a level of categorization or class somewhat above the direct level of what we might term species, or in the language of Aristotle, things which differ essentially in their form. A cow or ox (βους) is the same genus as a mouse, though both are essentially different. Interestingly, Aristotle seems to describe a method which ascribes a particular genus to a thing quite independent of whether or not another thing shares the same genus. He says both that, “connected with the genus,” is the question of whether a thing is in the same genus as another thing, and also that, “having said that ‘animal’ is the genus of human, and likewise the genus of cow, we shall have said that they are in the same genus.” The impression given is that these twin assessments of two distinct things are conducted separately, and upon reflection, when it turns out man and cow are both “animal,” then we sweep them into the “same genus.” In my view though, it would appear that a comparison is made from the outset, that one is consciously aware of comparing one thing to another, rather than as an independent assessment. Perhaps this is what Aristotle means. For instance he may be pointing out what is rather obvious, but he states the obvious nonetheless, with the goal of prompting us to have an eye on whether something is of the same genus as another thing. The last sentence in this section seems to support this view, as there is language of proof. Thus, whether or not something is of one genus or another, is helpful in proposing or refuting an argument, one of the chief reason the Topica was written. 2

1 Γένος δ’ ἐστὶ τὸ κατὰ πλειόνων καὶ διαφερόντων τῷ
εἴδει ἐν τῷ τί ἐστι κατηγορούμενον. ἐν τῷ τί ἐστι δὲ κατηγο-
ρεῖσθαι τὰ τοιαῦτα λεγέσθω ὅσα ἁρμόττει ἀποδοῦναι ἐρω-
τηθέντα τί ἐστι τὸ προκείμενον· καθάπερ ἐπὶ τοῦ ἀνθρώπου
ἁρμόττει, ἐρωτηθέντα τί ἐστιν, εἰπεῖν ὅτι ζῷον. γενικὸν δὲ (35)
καὶ τὸ πότερον ἐν τῷ αὐτῷ γένει ἄλλο ἄλλῳ ἢ ἐν ἑτέρῳ·
καὶ γὰρ τὸ τοιοῦτον ὑπὸ τὴν αὐτὴν μέθοδον πίπτει τῷ γένει.
διαλεχθέντες γὰρ ὅτι τὸ ζῷον γένος τοῦ ἀνθρώπου, ὁμοίως
δὲ καὶ τοῦ βοός, διειλεγμένοι ἐσόμεθα ὅτι ἐν τῷ αὐτῷ
(102b.) γένει· ἐὰν δὲ τοῦ μὲν ἑτέρου δείξωμεν ὅτι γένος ἐστί, τοῦ
δὲ ἑτέρου ὅτι οὐκ ἔστι, διειλεγμένοι ἐσόμεθα ὅτι οὐκ ἐν τῷ
αὐτῷ γένει ταῦτ’ ἐστίν.

2 See 101a25: The three uses of the treatise are for mental training, conversations and philosophical knowledge/science.