Selection from St. Thomas Aquinas On Being and Essence
Trans. Armand Maurer, C.S.B., cc. 1-11
1. A small mistake in the beginning is a big one in the end, according to the Philosopher in the first book of On the Heavens and the Earth. And as Ibn-Sînâ says in the beginning of his Metaphysics, being and essence are what is first conceived by the intellect.
2. Thus, to avoid making mistakes out of ignorance of them, and to become familiar with the difficulties they entail, we must point out what is signified by the words “being” and “essence,” and how they are found in diverse things, and how they are related to the logical intentions, genus, species, and difference.
3. Since we ought to acquire knowledge of what is simple from what is composed, and come to what is prior from what is posterior, so that, beginning with what is easier, we may progress more suitably in learning; we ought proceed from the meaning of the word “being” to that of the word “essence.”
4. We should notice, therefore, that the word “being,” taken without qualifiers, has two uses, as the Philosopher says in the fifth book of the Metaphysics. (1) In one way, it is used apropos of what is divided into the ten genera; (2) in another way, it is used to signify the truth of propositions. The difference between the two is that in the second way everything about which we can form an affirmative proposition can be called a being, even though it posits nothing in reality. It is in this way that privations and negations are called beings; for we say that affirmation is opposed to negation, and that blindness is in the eye. In the first way, however, only what posits something in reality can be called a being. In the first way, therefore, blindness and the like are not beings.
5. So, the word “essence” is not taken from the word “being” used in the second way; for some things which do not have an essence are called beings in this way as is clear in the case of privations. Rather, the word “essence” is taken from the word “being” used in the first way. It is for this reason that the Commentator says in the same place that the word “being” used in the first way is what signifies the essence of a real thing.
6. And because the word “being” used in this way is used apropos of what is divided into the ten genera, as we have said, the word “essence” must signify something common to all natures, by means of which (nature) diverse beings are placed into diverse genera and species; as, for example, humanity is the essence of man, and so with other things.
7. And because that by which a real thing is constituted in its proper genus or species is what is signified by the definition expressing what the real thing is, philosophers sometimes use the word “quiddity” for the word “essence.” This is what the Philosopher often calls what something was to be, i.e., that by which it belongs to something to be what it is.
8. It is also called form, in the sense in which the word “form” signifies the full determination of each real thing, as Ibn-Sînâ says in the second book of his Metaphysics.
9. Further, it is given another name, nature, taking the word “nature” in the first of the four ways given by Boethius in his book On the Two Natures. In this way, whatever can in any way be grasped by the intellect is called a nature. For a real thing is not intelligible except through its definition and essence.
10. The Philosopher, too, says in the fifth book of the Metaphysics that every substance is a nature. But the word “nature” taken in this way appears to signify the essence of a real thing according as it has an ordering to the thing’s proper operation; and no real thing lacks a proper operation.
11. The name “quiddity,” however, is taken from the fact that what is signified by the definition is the essence. But it is called essence from the fact that through it and in it a real being has existence.