To the former contention it might be objected that what individuates the specific nature cannot be conceived as anything _positive_, superadded to this nature: it cannot be anything _accidental_ to the latter, for if it were, the individual would be only an accidental unity, a “_unum per accidens_” and would be constituted by an accident, which we have seen to be inadmissible; nor, on the other hand, can it be anything _essential_ to the specific nature, for if it were, then individuals should be capable of adequate essential definition, and furthermore the definition of the specific nature would not really give the whole essence or _quidditas_ of the individuals—two consequences which are commonly rejected by all scholastics. To this, however, it is replied that the principle of individuation is something essential to the specific nature in the sense that it _is_ something intrinsic to, and really identical with, the whole real substance or entity of this nature, though not involved in the abstract concept by the analysis of which we reach the definition or _quidditas_ of this nature. What individuates Socrates is certainly essential to Socrates, and is therefore really identical with his human nature; it is intrinsic to the human nature in him, a mode or aspect of his human substance; yet it does not enter into the definition of his nature—“_animal rationale_”—for such definition abstracts from individuality. When, therefore, we say that definition of the specific nature gives the whole _essence_ of an individual, we mean that it gives explicitly the abstract (specific) essence, not the individuality which is really identical with this, nor, therefore, the whole substantial reality of the individual. We give different answers to the questions, “What is Socrates?” and “Who is Socrates?” The answer to the former question—a “man,” or a “rational animal”—gives the “essence,” but not explicitly the whole substantial reality of the individual, this remaining incapable of adequate conceptual analysis. The latter question we answer by giving the notes that _reveal_ individuality. These, of course, are “accidental” in the strict sense. But even the principles which constitute the individuality of separate individuals of the same species, and which differentiate these individuals numerically from one another, we do not describe as _essential_ differences, whereas we do describe specific and generic differences as _essential_. The reason of this is that the latter are abstract, universal, conceptual, amenable to intellectual analysis, scientifically important, while the former are just the reverse; the universal differences alone are principles about which we can have scientific knowledge, for “all science is of the abstract and universal”;(147) and this is what we have in mind when we describe them as “essential” or “formal,” and individual differences as “entitative” or “material”.