Aristotle, Metaphysics

[1058a] [1] and which is not accidentally differentiated, whether regarded as matter or otherwise.For not only must the common quality belong to both, e.g., that they are both animals, but the very animality of each must be different; e.g., in one case it must be equinity and in the other humanity. Hence the common quality must for one be other in species than that which it is for the other. They must be, then, of their very nature, the one this kind of animal, and the other that ; e.g., the one a horse and the other a man.Therefore this difference must be "otherness of genus" (I say "otherness of genus" because by "difference of genus" I mean an otherness which makes the genus itself other); this, then, will be a form of contrariety. This is obvious by induction.¹ For all differentiation is by opposites, and we have shown² that contraries are in the same genus, because contrariety was shown to be complete difference. But difference in species is always difference from something in respect of something; therefore this is the same thing, i.e. the genus, for both.(Hence too all contraries which differ in species but not in genus are in the same line of predication,³ and are other than each other in the highest degree; for their difference is complete, and they cannot come into existence simultaneously.) Hence the difference is a form of contrariety.

To be "other in species," then, means this: to be in the same genus and involve contrariety, while being indivisible(and "the same in species" applies to all things which do not involve contrariety, while being indivisible); [20] for it is in the course of differentiation and in the intermediate terms that contrariety appears, before we come to the indivisibles.⁴ Thus it is evident that in relation to what is called genus no species is either the same or other in species (and this is as it should be, for the matter is disclosed by negation, and the genus is the matter of that of which it is predicated as genus; not in the sense in which we speak of the genus or clan of the Heraclidae,⁵ but as we speak of a genus in nature); nor yet in relation to things which are not in the same genus. From the latter it will differ in genus, but in species from things which are in the same genus. For the difference of things which differ in species must be a contrariety; and this belongs only to things which are in the same genus.

The question might be raised as to why woman does not differ in species from man, seeing that female is contrary to male, and difference is contrariety; and why a female and a male animal are not other in species, although this difference belongs to "animal" per se, and not as whiteness or blackness does; "male" and "female" belong to it qua animal.This problem is practically the same as "why does one kind of contrariety (e.g. "footed" and "winged") make things other in species, while another (e.g. whiteness and blackness) does not?" The answer may be that in the one case the attributes are peculiar to the genus, and in the other they are less so;