The philosophers’ proof for tawḥīd (divine oneness) is built upon precisely this kind of confusion and equivocation. Thus, when the questioner says: “Things that differ in their individual essences yet agree in some constitutive element must fall under one of four alternatives: either (i) what they share necessarily follows from what distinguishes them, or (ii) the reverse, or (iii) what they share is merely incidental to what distinguishes them, or (iv) the reverse”—this entire formulation is of the same order. The first alternative is commonly illustrated by diverse species: their quiddity and genus are said to be concomitant to them, just as “man” and “horse” and the like are distinct species while “animal” is deemed necessary to each of them.
We reply: concrete individuals exist only as the very individuals they are within their species. In external reality there is no such thing as the species “man” in the abstract, nor the species “horse” in the abstract; rather, what exists are the individual members of the species, such as this particular horse and this particular man. Accordingly, you say that man and horse differ by virtue of their species, yet agree in something necessary to both, namely animality.
It will then be asked: by saying that they differ in their essences, do you mean the essences of their individual tokens or the essences of their species? If you intend the very entities (1) of the two individuals, then that wherein they agree—animality—is itself distinguished in each of them from the other by its own individuation; for the animality of this man is, in its very self, distinct from the animality of this horse. The one is not identical with the other. Just as the humanity of this man is, by individuation, other than the horseness (2) of this horse, so too the animality of this man is, by individuation, other than the animality of this horse. Nevertheless, animality resembles animality, whereas horseness contrasts with humanity; thus each of them resembles the other in one respect and differs from him in another. Within each there is an attribute by which he resembles the other, and within each there is an attribute by which he differs from the other.