Having seen what the term essence signifies in composite substances, we ought next see in what way essence is related to the logical intentions of genus, species, and difference. Since that to which the intentions of genus or species or difference is appropriate is predicated of this signate singular, it is impossible that a universal intention, like that of the species or genus, should be appropriate to the essence if the genus or species is signified as a part, as in the term humanity or animality. Thus, Avicenna says, Metaphysicae V, cap. 6, that rationality is not the difference but the principle of the difference. For the same reason, humanity is not a species, and animality is not a genus. Similarly, we cannot say that the intention of species or genus is appropriate to the essence as to a certain thing existing beyond singulars, as the Platonists used to suppose, for then the species and the genus would not be predicated of an individual: we surely cannot say that Socrates is something that is separated from him, nor would that separate thing advance our knowledge of this singular thing. And so the only remaining possibility is that the intention of genus or species is appropriate to the essence as the essence is signified as a whole, as the term man or animal implicitly and indistinctly contains the whole that is in the individual.
The nature, however, or the essence thus understood can be considered in two ways. First, we can consider it according to its proper notion, and this is to consider it absolutely. In this way, nothing is true of the essence except what pertains to it absolutely: thus everything else that may be attributed to it will be attributed falsely. For example, to man, in that which he is a man, pertains animal and rational and the other things that fall in his definition; white or black or whatever else of this kind that is not in the notion of humanity does not pertain to man in that which he is a man. Hence, if it is asked whether this nature, considered in this way, can be said to be one or many, we should concede neither alternative, for both are beyond the concept of humanity, and either may befall the conception of man. If plurality were in the concept of this nature, it could never be one, but nevertheless it is one as it exists in Socrates.
Similarly, if unity were in the notion of this nature, then it would be one and the same in Socrates and Plato, and it could not be made many in the many individuals. Second, we can also consider the existence the essence has in this thing or in that: in this way something can be predicated of the essence accidentally by reason of what the essence is in, as when we say that man is white because Socrates is white, although this does not pertain to man in that which he is a man.
The nature considered in this way, however, has a double existence.
It exists in singulars on the one hand, and in the soul on the other, and from each of these there follow accidents. In singulars, furthermore, the essence has a multiple existence according to the multiplicity of singulars.
Nevertheless, if we consider the essence in the first, or absolute, sense, none of these pertain to the essence. For it is false to say that the essence of man, considered absolutely, has existence in this singular, because if existence in this singular pertained to man insofar as he is man, man would never exist outside this singular. Similarly, if it pertained to man insofar as he is man not to exist in this singular, then the essence would never exist in the singular. But it is true to say that man, but not insofar as he is man, has whatever may be in this singular or in that one, or else in the soul. Therefore, the nature of man considered absolutely abstracts from every existence, though it does not exclude the existence of anything either. And the nature thus considered is the one predicated of each individual.
