One might raise the question, why woman does not differ from man in species, given that male and female differ from each other as being contrary; nor yet do male animal and female animal differ in species, though this difference belong substantially to animal and not as it is with paleness and darkness, but they belong to animal qua animal.
This question is almost the same as that asking, why one contrariety makes them different in species and another does not, as for instance pedestrian and winged do, but paleness and darkness do not. Or is it that the former are affections peculiar to the genus, while the latter are less so?
And since formula is on the one hand and matter on the other (each thing is composed of form and matter, and the form is a formula), contrarieties which are within the formula make a difference in species, while those which are included with matter do not (pale in pale man is not predicated of man’s formula but it is added in pale man’s formula when being predicated of man, since man underlies in pale man as its matter).
This is the reason why paleness makes no difference in man, neither does darkness; nor is there a difference between the pale man and the dark man in species, not even if it is replaced by a name. For man underlies as matter, and matter produce no difference, since men are not species of man because of itself, though the flesh and bones of which this man consists are different from those of which that man consists; but the compound is different, yet not in species because there is no contrariety in the formula; and this compound is the last in division individual species.
Thus Callias is a formula (form) together with matter; and consequently pale man too, insofar as Callias is pale; man, then, is pale accidentally. But also a brazen circle and a wooden circle do not differ from each other, nor do a brazen triangle and a wooden circle differ in species because of the matter, but because of the fact that there is a contrariety in the formula.
Does the matter, therefore, not produce otherness in species insofar as it is in a certain way different, or is there a sense in which it does? For why is this horse different in species from that man? Yet their formulae are included with their matter.
Or is there a contrariety in the formula? Undoubtedly there is a contrariety in formula between pale man and dark horse. And it is certainly in species, not on the ground that the one is pale and the other dark, for also in the case where they were both pale, yet they would be different in species.
However, male and female are affections peculiar to animal, but not in virtue of its essence but in matter and body. This is why the same seed become female or male by being affected somehow.
We have stated, then, what otherness in species means, and why some things are different in species and others are not.
Bibliography: Aristotle, Metaphysics (1058a.29 to 1058b.25)
Translation: George Kotsalis