The Daily Logos XIII
“Note that sometimes a thing can be although it is not, whereas sometimes a thing simply is. That which can be is said to be in potentiality; that which already is is said to be actually. But existence is twofold, namely essential or substantial existence; but there is also accidental being, as when a man is white, and this is to be only in a secondary sense.
“Something can be in potency to either kind of existence, as sperm and menstrual blood are potentially man, and a man is potentially white. Both that which is potentially a substantial being and that which is potentially some accidental being can be called matter, as sperm is the matter of man and man is the matter of whiteness, but they differ in this that the matter which is in potency to substantial being is called the matter from which, and that which is in potency to accidental being is called the matter in which.
“Again, properly speaking, what is in potency to accidental being is called a subject, while what is in potency to substantial being is properly called matter. A sign that what is in potency to accidental being is called a subject is the fact that accidents are said to be in a subject, whereas substantial form is not said to be in a subject. Matter differs from subject in that the subject does not receive existence from that which advenes to it, but has complete existence in itself, as man does not receive existence from whiteness. Matter however has existence from that which advenes to it; of itself it has incomplete existence. Thus, simply speaking, form gives existence to matter, but the subject [gives it] to accident. Sometimes, however, these terms are used interchangeably, matter for subject, and vice versa.
“Just as anything that is in potency can be called matter, so anything from which something has existence, whatever kind of being it is, whether substantial or accidental, can be called form: man when he is potentially white comes to be actually white through whiteness, and sperm when it is potentially man becomes man actually through the soul. And because form causes a thing to be in act, form is said to be act, but substantial form causes actual substantial existence, and what causes actual accidental existence is called accidental form.”
-St. Thomas Aquinas, De Principiis Naturae
Though at times perfectly lucid, some of the Angelic Doctor’s writings were not formed in the clear structure that marks much of the Summa Contra Gentiles and the even clearer dialectical organization of the Summa Theologiae. This selection, from the beginning of De Principiis Naturae (On the Principles of Nature), discusses the difference between substance and accidents as according both their simple being, such as it is, and their being in a particular way; as both esse and esse aliquid. As Aquinas explains, both substantial and accidental being can be said to be matter, insofar as both have some material existence (not necessarily corporeal—it is important to recall that Aquinas often uses language analogously). Nonetheless, this materiality in something substantial results from a higher form or essence; it is a “from-which, for-the-sake-of-which.”Accidental being, as a particular manifestation of some substantial being, has the particularities of its actual existence only within some substantial being; thus its form is a form only secondarily speaking, its existence inhering in some yet greater substantial being. Understanding of these differences is critical to understanding the distinction between man’s essence and existence.