Disputationes metaphysicae

1. The description of form in genus.—From what has been said of the existence of substantial form and of the mode in which it comes about, it can easily be understood what it itself is and what mode of causing it has; but we can comprehend each in this brief description: form is a certain simple and incomplete substance which, as the act of matter, constitutes with it the essence of the composite substance. In which description substance is posited in place of genus, in the way in which it is distinguished by the Philosopher into matter, form and composite. And by that particle this form is distinguished from accidental ones, which are not substances; it can also be distinguished from substantial modes; for, although these are reduced to the predicament of substance, they are nevertheless not substances in that propriety; whence neither are they enumerated by Aristotle in that partition, because they are not properly substantial entities, as is the form, which by this reason is called substance. But simple and incomplete is added in order that by the former particle it may be separated from composite substance, but by the latter from separated substances, which sometimes are wont to be called assisting forms, as those which move the heavenly bodies, which in truth do not exercise the function of formal cause but some efficiency; wherefore they are not substantial forms as we are now speaking of them.

2. How the form is said to be the act of a physical body.—The remaining part of the definition distinguishes the form from matter, which too is a simple and incomplete substance, yet as potency; but the form as the primary act of that potency. And therefore Aristotle, De Anima II, defined the soul to be the act of a physical body, etc. Which definition can easily be accommodated to form in common if by the name of act we understand substantial act, as it must be understood, and only say that form is the act of a physical body, taking away the remaining part which separates the soul from the forms of inanimate things. But by physical body we can understand either matter itself, in so far as it is the other part of the natural composite, which, in order that it be informed by the form, is always supposed in the order of nature in its genus of cause affected by quantity and some proximate dispositions, and by that reason can be called a physical body; or we can also understand by physical body the natural being itself which consists of matter and form, whose act the form is said to be as constituting it; in which way Aristotle too defined it to be the act of the living, or that by which we live. Each relationship, then, we have explained in the said part of our description; for the substantial form is so an incomplete substance that it is the act of matter—act (I say) either actually informing or by its nature instituted to inform matter, for words (as they say) in definitions express aptitude, and so the rational soul is comprehended, even if it is separated from the body. And hence it consequently has that with matter it composes the essence of the natural being, which is composite substance; and therefore Aristotle, Physics II, c. 3, says that form is the notion of the essence or quiddity, that is, which completes the essence of the natural composite and distinguishes it essentially and quidditatively from others.

3. And hence the causality of form too has been explained; for, because the form is essentially act, therefore it intrinsically includes a transcendental relationship to that whose act it is and to the actuation which it exercises in it; and therefore the notion of form can scarcely be declared except through the relationship to its causality. In which we can consider those four things which we expect in every cause, namely the formal principle of causing, the condition necessary for causing, the causation itself, and the effect which is caused, all of which are to be explained one by one, as we did in matter, although, the things said there being supposed, they can be dispatched more briefly here.