That which is “other in species” than something else is “other” in respect of something and that something must apply to both. E.g., if an animal is other in species than something else, they must both be animals. Hence things which are other in species must be in the same genus. The sort of thing I mean by “genus” is that in virtue of which two things are both called the same one thing; 1058aand 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); 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; 1058band since one element is formula and the other matter, contrarieties in the formula produce difference in species, but contrarieties in the concrete whole do not. Hence the whiteness or blackness of a man does not produce this, nor is there any specific difference between a white man and a black man; not even if one term is assigned to each. For we are now regarding “man” as matter, and matter does not produce difference; and for this reason, too, individual men are not species of “man,” although the flesh and bones of which this and that man consist are different. The concrete whole is “other,” but not “other in species,” because there is no contrariety in the formula, and this is the ultimate indivisible species. But Callias is definition and matter. Then so too is “white man,” because it is the individual, Callias, who is white. Hence “man” is only white accidentally. Again, a bronze circle and a wooden one do not differ in species; and a bronze triangle and a wooden circle differ in species not because of their matter, but because there is contrariety in their formulae. But does not matter, when it is “other” in a particular way, make things “other in species”? Probably there is a sense in which it does. Otherwise why is this particular horse “other in species” than this particular man, although the definitions involve matter? Surely it is because there is contrariety in the definition, for so there also is in “white man” and “black horse”; and it is a contrariety in species, but not because one is white and the other black; for even if they had both been white, they would still be “other in species.” “Male” and “female” are attributes peculiar to the animal, but not in virtue of its substance; they ar material or physical. Hence the same semen may, as the result of some modification, become either female or male. We have now stated what “to be other in species” means, and why some things differ in species and others do not.

Since contraries are other in form,⁶ and “the perishable” and “imperishable” are contraries (for privation is a definite incapacity), “the perishable” must be “other in kind” than “the imperishable.” But so far we have spoken only of the universal terms; and so it might appear to be unnecessary that anything perishable and imperishable should be “other in form,” just as in the case of white and black. For the same thing may be both at the same time, if it is a universal (e.g, “man” may be both white and black); and it may still be both if it is a particular, for the same person may be white and black, although not at the same time. Yet white is contrary to black. But although some contraries (e.g. those which we have just mentioned, and many others) can belong to certain things accidentally, others cannot; 1059aand this applies to “the perishable” and “the imperishable.” Nothing is accidentally perishable; for that which is accidental may not be applicable; but perishability is an attribute which applies necessarily when it is applicable at all. Otherwise one and the same thing will be imperishable as well as perishable, if it is possible for perishability not to apply to it. Thus perishability must be either the substance or in the substance of every perishable thing. The same argument also applies to the imperishable; for both perishability and imperishability are attributes which are necessarily applicable. Hence the characteristics in respect of which and in direct consequence of which one thing is perishable and another imperishable are opposed; and therefore they must be other in kind. Thus it is obvious that there cannot be Forms such as some thinkers maintain; for then there would be both a perishable and an imperishable “man.” ⁷ Yet the Forms are said to be the same in species as the particulars, and not merely to share a common predicate with them; but things which are other in genus differ more widely than things which are other in species.