Selection from Etienne Gilson, The Christian Philosophy of St. Thomas Aquinas

(New York: Random House, 1956), 29-30.

Beginning as St. Thomas himself does, with the entia, or beings which come to us in sense experience, we shall designate them by the term "substances." Each substance forms a complete whole which has a structure that we shall analyze and which constitutes an ontological unit Capable of being given a definition. In so far as substance can be conceived and defined, it is called "essence." Essence, therefore, is only substance as susceptible of definition. To be exact, the essence is what the definition says the substance is. This is why, following terminology of Aristotle, St. Thomas introduces a third term into his description of the real. To signify what a substance is, is to reply to the question quid sit (what is it?), and so, to the extent that it is expressed in the definition, such is the first group of terms which we shall be constantly using. They are too closely related not to be sometimes employed one for the other, but we should know, whenever it becomes necessary, how to reduce them to their primary meaning.

Since essence is substance in so far as it is knowable, it ought to include the latter in its complete being and not only one or other of the elements of which it is composed. Substance is sometimes defined as "a being by itself." This is not inexact, but is not the whole truth, and it is in completing this formula as it ought to be that we discover the proper sense of the notion of essence. Indeed, substance is not conceivable, and, consequently, is not definable, unless we think of it as a given determined substance. This is why "a being by itself" cannot exist without a complementary determination. The essence alone furnishes this determination. Substance, therefore, must be defined as an essence or quiddity existing by itself in virtue of its own act of being.

We can understand it better if we examine the meaning of the formula "being by itself." Let us take any substance whatsoever, a man, for example. He is said to exist by himself because he constitutes an ontological unit distinct from any other and contains in himself all the determinations necessary for his existence. However, all these determinations do not exist in him by the same title nor in the same manner. There are first of all those without which he could not be called a man and which permit us so to call him. Definitions express these determinations. In the present instance, this substance is a man because it is an animal endowed with reason. Let us assume that such a substance has been concretely realized. Then all complementary determinations will be realized at the same time, and by means of it. Because he is an animal, a man must have a certain color and shape. He will necessarily occupy a certain place in space and a certain relative position. We put he name substance on the subject of these complementary determinations which themselves are called accidents. Although, within our experience, there are no substances without accidents any more than there are accidents without substance, still it is the accidents which belong to the substance, not the substance to the accidents.