As the Philosopher says in V Metaphysicae cap. 7 (1017a22-35), being has two senses. In one sense, being signifies that which is divided into the ten categories; in another sense, that which signifies the truth of propositions. The difference between these is that, in the second sense, anything can be called a being about which an affirmative proposition can be formed, even if the thing posits nothing in reality. In this way, privations and negations are called beings, as when we say that affirmation is opposed to negation, or that blindness is in the eye. But in the first sense, nothing can be called a being unless it posits something in reality, and thus in this first sense blindness and similar things are not beings.
The term essence is not taken from being in the second sense, for in this sense some things are called beings that have no essence, as is clear with privations. Rather, the term essence is taken from being in the first sense. Thus in Metaphysicae V, com. 14, the Commentator explains the cited text from Aristotle by saying that being, in the first sense, is what signifies the essence of a thing. And since, as said above, being in this sense is divided into the ten categories, essence signifies something common to all natures through which the various beings are placed in the various genera and species, as humanity is the essence of man, and so on.
Since that through which a thing is constituted in its proper genus or species is what is signified by the definition indicating what the thing is, philosophers introduced the term quiddity to mean the same as the term essence; and this is the same thing that the Philosopher frequently terms what it is to be a thing, that is, that through which something has being as a particular kind of thing. Essence is also called form, for the certitude of every thing is signified through its form, as Avicenna says in his Metaphysicae I, cap. 6. The same thing is also called nature, taking nature in the first of the four senses that Boethius distinguishes in his book De Persona et Duabus Naturis cap. 1 (PL 64, 1341B), in the sense, in other words, that nature is what we call everything that can in any way be captured by the intellect, for a thing is not intelligible except through its definition and essence. And so the Philosopher says in V Metaphysicae cap.
4 (1014b36) that every substance is a nature. But the term nature used in this way seems to signify the essence of a thing as it is ordered to the proper operation of the thing, for no thing is without its proper operation.
The term quiddity, surely, is taken from the fact that this is what is signified by the definition. But the same thing is called essence because the being has existence through it and in it.
But because being is absolutely and primarily said of substances, and only secondarily and in a certain sense said of accidents, essence too is properly and truly in substances and is in accidents only in a certain way and in a certain sense. Now some substances are simple and some are composite, and essence is in both, though in the simple substances in a truer and more noble way, as these have existence in a nobler way: indeed, the simple substances are the cause of the composite ones, or at least this is true with respect to the first simple substance, which is God.
But because the essences of these substances are more hidden from us, we ought to begin with the essences of composite substances, as learning is easier when we begin with the easier things.
