De Rervm Natvra Constantivm Scientia, Principiis Et Affectionibvs Commvnibvs, Dispvtatio

continues: ...nature of a substance, which is defined as that which exists through the diffusion of itself into another, lacking the power of subsisting of itself. Even if it is difficult to undermine this by reason, it is of such a kind that it relies on no argument, and for that reason it easily collapses of itself. The other [method] is to say that that which subsists of itself is separated from every essential composition, and all remaining things that are present are accidents.

XXXIIX. The decrees of the faith argue against this method, as they teach that the substance of man is composed of soul and body, and that beyond this composite substance, there are many accidents in man. Since we suspect this twofold composition was necessary in man, both on account of his operations and because he is subject to corruption, and when we perceive the same operations—or those not greatly differing—in other things, and observe that they are all subject to corruption, we rightly conclude that this twofold composition of accident and substance has a place in all substances, and consequently, that one form belongs to the substance, another to the accident.

XXXIX. Substantial form (insofar as that which belongs to an accident pertains to the Metaphysician) is defined as an imperfect substance, constituting the principle of generation with matter to form a composite, giving the same [composite] existence and operation. Except for one thing, all else is understood from the preceding points made in the chapter on nature and matter; that one thing is what is understood by the name "composite."

XL. The answer is that it is understood as that which is composed of several parts, and is a single thing in the genus of substance. And it is called a "single thing of itself" for three reasons: firstly, because its parts are of one genus, all imperfect in the same genus in which the whole is contained; secondly, because an order exists among them such that one, by perfecting another, and the other by receiving it, constitute a perfect and absolute entity in the same genus; thirdly, because it is that to which operations comprehended by a certain species correspond, which no part can perform perfectly by itself.

XLI. From the explained definition of form, it follows first that the composite is more excellent than the form, for the whole contains the form and matter as well. Even if it were concluded by this argument only that the composite is more excellent in greater perfection according to extension, this is sufficient to prove the point: that the composite is simply more excellent than the form. Secondly, it follows that the form is of itself indivisible by the division of extension or quantity, because it is a part of the substance. Yet, one form is divisible by accident, another is not divisible even by accident. Forms divisible by accident are those which can be divided into parts by the division of quantity; such forms cannot exist except in the actual extension of their parts, and for that reason, they demand by their own nature the quantity by which they are actually extended.