In De ente et essentia D. Thomae Aquinatis commentaria

It is explained how accidents have an essence, how they are distinguished from substance and from one another, and how they are related to logical intentions.

It remains now to see how essence is in accidents; how it is in all substances has been said. Since, as was said, essence is signified by definition, they must have essence in the same way in which they have definition. They have an incomplete definition, because they cannot be defined unless a subject is placed in their definition. This is because they do not have absolute existence in themselves apart from a subject; but just as substantial existence remains from form and matter when they are composed, so accidental existence remains from accident and subject when the accident comes to the subject. Hence neither substantial form nor matter has a complete essence: in the definition of substantial form that of which it is the form must be posited, and in the definition of matter its form must be posited. This is because neither is a complete species in the genus of being, but each is part of a species. Thus neither can be defined without adding the other, as is said in Metaphysics VII, text 45. Accidents, however, do not have a complete essence, nor are they part of a complete essence, because they do not constitute a species in the genus of substance. Hence, just as they are beings according to a qualification, so they have an essence according to a qualification.

Nevertheless, there is this difference between substantial and accidental forms: just as substantial form, joined to matter, causes the substantial existence of the composite, so accidental form causes a certain second existence, without which the subsisting thing can still be understood to exist, just as the first can be understood without the second, or a predicate without a subject. Hence accident and subject do not make one per se, but one per accidens. Their conjunction therefore does not produce a certain essence, as the conjunction of form with matter does; for this reason an accident has neither the character of a complete essence nor of a part of a complete essence, but, just as it is a being according to a qualification, so it has an essence according to a qualification.

In this third treatise, contained in one chapter, accidents are determined so that nothing within the breadth of being remains untouched. It is divided into three parts: in the first, what essence signifies in accidents is explained; in the second, the diversity of accidents from substance and from one another; in the third, how they stand in relation to logical intentions. He therefore explains the first with this conclusion. Accidents have an incomplete essence. It is proved thus: whatever have an incomplete definition have an incomplete essence; accidents have an incomplete definition; therefore they have an incomplete essence. The major is clear from the fact that essence is what is signified by definition, as was said at the beginning of the book.

The proof of the minor is this: whatever are necessarily defined through something outside their essence have an incomplete definition; for the definition of such things is not completed through intrinsic principles but needs an extrinsic addition. But accidents are necessarily defined through something outside their essence; their subject must be placed in their definition, as is said in Metaphysics VII, text 15; therefore accidents have an incomplete definition.

To manifest the minor more clearly, St. Thomas in the text compares accidents with substantial forms according to their agreement and difference. It will thereby be clear how accidents have an incomplete essence.

He says that accidents agree with substantial forms in three respects. First, neither has absolute existence independent of a subject or receptive principle. Just as existence common to both flows from substantial form joined to matter, so existence common to both remains from the conjunction of accident with subject: from the conjunction of whiteness with snow there arises the existence by which whiteness exists and snow is white.

Second, they agree in that neither has a complete essence, because the essence of each is essentially referred to another as receptive. Substantial form is essentially an entity perfecting matter, while an accident is essentially an entity qualifying a subject.

Third, they agree in that neither has a complete definition in its own terms, but only through an addition. Just as the subject of an accident, which lies outside its genus, must be placed in its definition, so the receptive principle of substantial form—matter, which lies outside its proper genus—must be placed in the definition of substantial form. For the genus of form is act, as is clear in On the Soul II; while matter, since being is divided into act and potency, is clearly contained under potency and consequently lies outside act, which is the genus of form. Aristotle showed this when defining soul, placing in its definition an organic physical body, which is the proper matter of soul insofar as soul is the perfection of a physical body. He added this so that you would not wonder why matter is not placed in the definition of soul, although soul is a substantial form, but a physical body is. The reason is that the physicist does not consider soul as substantial form, but as soul, which as such presupposes a physical body as its proper matter. Hence it follows that to consider the intellectual soul as such according to its complete quiddity is not the work of the physicist, as Aristotle also teaches in Book I of On the Parts of Animals, chapter I.

They differ in five respects. First, the receptive principle of substantial form does not have existence in itself without the existence of the form, and in this it conforms to its form. Just as substantial form has no existence apart from its receptive principle, so neither does its receptive principle have existence apart from its form. But the receptive principle of an accident has existence in itself apart from the existence of its accident. The reason is that the receptive principle of substantial form is a being in pure potency, whereas the receptive principle of an accidental form has existence in act.

Second, they differ because from the conjunction of substantial form with its receptive principle there emanates the existence by which the thing itself subsists, or simply is. From the conjunction of accidental form with its subject flows the existence by which the thing is according to a qualification. This difference arises from the first: since the receptive principle of substantial form lacks existence in itself, the existence flowing from the form is primary; conversely, since the subject of an accident enjoys its own existence, by acquiring the accident it acquires such existence, not existence simply, which it already possesses. Thus the first existence can be understood without the second: substance can be separated from an accident, as is said in Metaphysics VII, text 4.

Third, they differ because from the composition of substantial form with its subject there results one per se. From the composition of accident with subject there results one only per accidens. This difference arises from the second: since from substantial form there results existence simply, by which the thing is per se, one simply must result, by which the thing is one per se, for being follows unity. But since subject and accident have two existences, they cannot make one per se for the same reason: unity follows being, and unity per se cannot follow being per accidens.

Fourth, they differ because from the conjunction of substantial form with matter there results a certain third essence, namely entity. From the conjunction of accident with subject no third essence results. This difference arises from the third: since essence is what is signified by definition, as the defined thing, and the defined thing must be one per se according to Aristotle in Metaphysics VII, text 16, it follows that, since accident and subject do not make one per se, no essence results; conversely, since substantial form and matter make one per se, a third essence results.

Fifth, they differ because, although substantial form is not a complete essence, it is nevertheless a part of a complete essence. An accident, however, is neither a complete essence nor a part of a complete essence. This fifth difference follows from the fourth. If form and matter make a certain third essence, substantial form will be a part of it. But if no essence results from accident and subject, the accident can be posited as a part of no complete essence. Thus an accident is an incomplete and qualified being and consequently has an incomplete essence and definition. Nor should it disturb you that substantial form has just been said not to be absolute from its receptive principle and not to have existence without matter, while above it was said that the intellectual soul remains independent of and really separated from matter. The latter is simply true. To verify what we have just said, it is enough that the intellectual soul essentially informs matter and is, in this respect, inseparable from it according to quidditative existence, as its definition shows; and that substantial form as such has no basis for being absolute from matter and independent of it according to actual existence, although this may belong to some substantial form for another reason, for example because it is intellectual. We are speaking of substantial form as the form of a part, as such.

Many doubts arise concerning these statements. Regarding the second agreement, it is doubted whether what is said there is true, namely, that accidental essences are essentially not absolute from a receptive principle. This doubt is accompanied by another concerning the third agreement: whether accidents are necessarily defined through a subject.

Questions also arise concerning the differences assigned. The first has a doubt concerning both members together with their root: whether substantial form is only the act of pure potency.

And whether accidental form is received only in a being in act.

Regarding the second difference, there is a doubt whether accidents have their own existence, really distinct from substantial existence.

There is again a doubt there: whether a substance subjected to an accident can be understood to exist without any accident at all.

If we wished to treat these six doubts at length, we would go beyond the limits of our purpose, since that belongs to another undertaking. But, so that we do not pass over them wholly without nourishment, something will be said about several of them in the order stated above, bringing forward both our foundations and those of our adversaries.