Aristotle, Metaphysics

[1030a] [1] but its essence is not "to be white." But is "to be X" an essence at all?Surely not. The essence is an individual type; but when a subject has something distinct from it predicated of it, it is not an individual type. E.g., "white man" is not an individual type; that is, assuming that individuality belongs only to substances. Hence essence belongs to all things the account of which is a definition.We have a definition, not if the name and the account signify the same (for then all accounts would be definitions; because any account can have a name, so that even "the Iliad " will be a definition), but if the account is of something primary. Such are all statements which do not involve the predication of one thing of another.Hence essence will belong to nothing except species of a genus, but to these only; for in these the predicate is not considered to be related to the subject by participation or affection, nor as an accident. But of everything else as well, if it has a name, there will be a formula of what it means—that X belongs to Y; or instead of a simple formula one more exact—but no definition, nor essence.

Or perhaps "definition," like the "what," has more than one sense. For the "what" in one sense means the substance and the individual, [20] and in another each one of the categories: quantity, quality, etc.Just as "is" applies to everything, although not in the same way, but primarily to one thing and secondarily to others; so "what it is" applies in an unqualified sense to substance, and to other things in a qualified sense. For we might ask also what quality "is," so that quality also is a "what it is"; not however without qualification, but just as in the case of not-being some say by a verbal quibble that not-being "is"—not in an unqualified sense, but "is" not-being—so too with quality.

Now although we must also consider how we should express ourselves in each particular case, it is still more important to consider what the facts are. Hence now, since the language which we are using is clear, similarly essence also will belong primarily and simply to substance, and secondarily to other things as well; just as the "what it is" is not essence simply, but the essence of a quality or quantity.For it must be either by equivocation that we say that these things are , or by adding and subtracting qualifications, as we say that the unknowable is known¹; since the truth is that we use the terms neither equivocally nor in the same sense, but just as we use the term "medical" in relation to one and the same thing;