In De ente et essentia D. Thomae Aquinatis commentaria

He shows what is signified by the name of being and essence.

He also gives various names that signify essence.

It should be known that, as the Philosopher says in V Metaphysicae, 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 propositions. The difference between these is that, in the second way, anything of which an affirmative proposition can be formed may be called being, 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 nothing can be called a being unless it posits something in reality. Hence blindness and the like are not beings in the first way. The name essence, therefore, is not taken from being in the second sense: for some things are called beings in this way that do not have an essence, as is clear in privations; rather, essence is taken from being in the first sense. Hence the Commentator says in the same place: “Being said in the first way is that which signifies the substance of a thing.” This is where the treatise begins, divided into two parts. In the first it is explained what is signified by the name of being and essence; in the second, how essence is found differently in different things. The first part is completed in one chapter, the second in six; and thus the entire treatise comprises seven chapters. The first of these, which we have before us, contains three parts: in the first it is explained what is signified by the name of being; in the second, what is signified by the name of essence; in the third, the difference among the names of essence itself is designated. The first is explained by this division. Being in itself is said in two ways: in one way, as divided into the ten genera; in another way, as signifying the truth of propositions, as Aristotle says in V Metaph. text. XIV. I said “being in itself” to distinguish it from being per accidens, aggregated from things of different genera, such as a white man; concerning this, see V Metaph. text. XII. He indicates that being in the first way and being in the second way differ in three respects. First, being in the first way is predicated only of things existing in the ten genera, whereas being in the second way is predicated of everything that can be the subject of an affirmative proposition. Secondly, they differ because being in the first way is the composite mentioned above, which in some fashion includes essence and being within itself, whereas being in the second way is not of this kind: for something can be a being in this way (for example, a negation or a chimera) without having any essence. From this follows the third difference: the name essence is taken from being in the first way, as from a composite more known to us, but not from being in the second way.

Concerning the first difference, note first that the truth of a proposition, which is being signified in the second way, is nothing other than a composition made in the intellect's second operation and conformed to the object. For example, in “Sortes est caecus,” the word “est” does not signify the inherence of blindness in Socrates, since blindness lacks all inherence, whereas inhering belongs to real accidents; rather, it signifies a composition made by the intellect, which through it conforms itself to the object—namely, to Socrates lacking the power of sight. Hence in V Metaph., in another wording, it is said that being signifying the truth of a proposition signifies that the proposition is true. What the first signification of being is will be seen below.

Note secondly that, although Socrates is blind without any intellect considering it, and through the intellect's consideration does not become more or less blind, nevertheless blindness, without any intellect considering it, has no being in Socrates; these two things stand together. This is explained as follows. For Socrates to be blind, precisely as such, is not for Socrates to have some substantial being, as is clear, nor an accidental being, because Socrates becomes blind solely through the absence of the power of sight, which adds nothing to Socrates. Hence blindness posits no being at all in Socrates; and because, without any consideration of the intellect, the power of sight is not present in Socrates, Socrates must be blind without any intellect considering it. The doubt in this matter arises because it is not rightly understood that to be blind is not to be something, but to lack the power of sight. For example, a ship lacks a helmsman without any intellect considering it; yet the absence of the helmsman posits no substantial or accidental being in the ship. Hence for a ship to be without a helmsman outside the soul is not to be something, but not to be steered. Privations and negations, however, acquire being and become beings because the intellect, understanding privations through habits and negations through affirmations, forms in itself in some way a certain likeness of the thing lacking something. For example, when the intellect forms in itself a certain likeness of a ship lacking a helmsman, which is this mental proposition, “A ship is without a helmsman,” it is not the presence of the helmsman—which outside the soul posits nothing in the soul—that becomes a being because the intellect has made it some term of the proposition; and since this being is in the soul and has no other being, it follows that negations and privations of this kind are beings only objectively in the soul. Thus their being is nothing other than being understood; in this way alone all beings of reason have being.

Concerning the third difference, it should be noted that, since names are signs of concepts, the order of naming follows the order of conceiving. Therefore, just as we derived knowledge of essence from being in the first way, as from a composite more known to us, so the name essence has its origin from being.

And because, as was 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 similarly in other cases.

In this second part it is explained what is imported by the name essence, in the following conclusion: Essence signifies something common to all the natures of all beings. He intimates its explanation as follows. Just as being signifies something common to all existing things, so essence, since it is taken from being, signifies something common to all genera and species. And just as being signifies that which is in every genus and species, so essence must signify that by which something is in any genus and species; for being and essence are related as that which and that by which. But since the natures of things are those by which things are placed in genera and species, it follows that essence signifies something common to all the natures of things. Hence, just as being is predicated of man, ox, and the like, which are in genera, so essence is predicated of humanity and bovinity, by which they are placed in genera. By the name essence, therefore, the nature of a thing is understood, just as by the name being, that which has that nature is understood.