Commentaria in Summam contra Gentiles, Liber II

Because it was said in the preceding conclusion that in intellectual substances there is a composition of essence and esse as of potency and act, someone might suppose that this composition is of the same kind as the composition of matter and form, since that too is from potency and act. Therefore, wishing to remove this doubt, Saint Thomas posits a manifold difference between the two compositions ¹.

I. The first of which is this: that matter is not the substance and essence of the thing, but a part of its essence.

Second, because esse is not the act of matter, so that it could be called that which is, but of the whole substance. For the substance itself is that which is: namely, insofar as it is signified in the concrete.

Third, because esse is not the form itself, but the form is related to esse itself as light to shining and whiteness to being white: that is, as the formal cause to its formal effect. — Esse is also related to form as act: because in composites of matter and form, therefore, the form is called the principle of being, because it is the complement of the substance, whose act is esse itself. — Hence neither matter nor form is that which is, nor is esse itself esse; but the form is that by which it is; and esse itself is that by which the substance is denominated a being ²; but the composite substance itself is that which is. In intellectual substances, however, the form is that which is, and esse is its act and that by which it is.

II. From these things it follows first that in intellectual beings there is only one composition of act and potency, namely, that of essence and esse. But in composites of matter and form there is a twofold composition: the first is that of the substance itself, which is composed of matter and form; the second is that of the substance already composed and its esse.

It follows, second, that the composition of act and potency is broader than the composition of form and matter. The sign of this is that matter and form divide material substance ³; but act and potency divide being in common (ens commune).

From which it follows, third, that those things which follow potency and act insofar as they are such are common to created material and immaterial things. But those things which are proper to matter and form insofar as they are such belong only to material substances.

III. Concerning what is said in the third difference ⁴, namely that form is that by which a thing is, and esse that by which substance is denominated a being, it must be noted that both form and esse are that by which the composite is, but in different ways. For form is that by which something is, as the formal principle, whose formal effect is esse: just as whiteness is that by which something is white. Hence Saint Thomas said that form can be called that by which a thing is, as the principle of essendi. Esse, however, is that by which substance is, as the act by which it is denominated a being. For although form makes its denomination insofar as it is form—for from soul something is called animate—it does not make the denomination according to this name "existing" or "being": for the animate thing is not denominated a being or existing from soul, but from esse itself. Form, therefore, is that by which something is, as the formal principle of its esse. Esse itself, however, is that by which something is, as the act from which something is formally denominated a being, or existing.

IV. It must also be noted that from the differences stated above there results another difference between the composition of essence and esse, and the composition of matter and form. For since it was said that matter is not the substance of the thing, which is its essence, but only a part of its substance, it follows that from the composition of matter and form some whole results: for from essential parts an essential whole results. But since the essence of a thing is not a part of its substance, but the whole, therefore from essence and esse no whole results, since from two acts no third thing that is one per se results; although one act can be united to another per se when one stands to the other as potency. Hence the composition of matter and form is called a composition from those things: whereas the composition of essence and esse is called a composition with those things, because essence composes with esse, and esse with essence. And if something is sometimes said to be composed of essence and esse, this is said improperly: properly it is said that in it there is a composition of essence with esse, and conversely of esse with essence.