De ente et essentia

It must be known, therefore, that, as the Philosopher says in Book V of the Metaphysics, being in itself is said in two ways: in one way, that which is divided into the ten genera; in another way, that which signifies the truth of a proposition. The difference between these, however, is that in the second way anything can be called being of which an affirmative proposition can be formed, even if it posits nothing in reality; in this way privations and negations are called beings. For we say that affirmation is opposed to negation, and that blindness is in the eye. But in the first way only that can be called being which posits something in reality; hence, in the first way, blindness and things of this kind are not beings.

The name “essence,” therefore, is not taken from being in the second sense just mentioned: for in this way some things are called beings that do not have an essence, as is clear in the case of privations; rather, essence is taken from being in the first sense. Hence the Commentator says in the same place that being said in the first way is that which signifies the essence of a thing. And since, as has been said, being in this sense is divided into the ten genera, essence must signify something common to all the natures through which diverse beings are placed in diverse genera and species, just as humanity is the essence of man, and likewise in other cases.

And since that through which a thing is constituted in its proper genus or species is what is signified by the definition stating what the thing is, it follows that philosophers change the name “essence” into the name “quiddity”; and this is also what the Philosopher frequently calls “that which it was to be,” that is, that through which something has the being of what it is. It is also called form, insofar as by form the determination of each thing is signified, as Avicenna says in Book II of his Metaphysics. It is also called nature by another name, taking nature according to the first of the four ways that Boethius assigns in the book On the Two Natures: namely, insofar as nature is called everything that can in some way be grasped by the intellect, for a thing is intelligible only through its definition and essence. Thus the Philosopher also says in Book V of the Metaphysics that every substance is a nature. Nevertheless, the name “nature,” taken in this way, seems to signify the essence of a thing insofar as it has an order to the thing’s proper operation, since no thing is without its proper operation; whereas the name “quiddity” is taken from the fact that it is signified through the definition. But “essence” is said insofar as through it and in it a being has being.

But since being is said absolutely and primarily of substances, and secondarily and, as it were, in a qualified sense of accidents, it follows that essence also is properly and truly in substances, but in accidents it is there in a certain way and in a qualified sense. Some substances, moreover, are simple and others composite, and essence is found in both; but in simple substances it is found in a truer and nobler way, just as they also have nobler being. For they are the cause of composite things—at least the first simple substance, which is God, is. But since the essences of those substances are more hidden from us, we must begin with the essences of composite substances, so that the instruction may proceed more suitably from what is easier.