Causality and the Metaphysics of Change
in Aristotle and St. Thomas Aquinas

by Mario Derksen

Extract from 

As we have seen already, Aristotle thought of all individual existing things as hylomorphic substances. A consequence of this view is that, for Aristotle, all individual substances are material; there is no such thing as an immaterial substance, since substances are necessarily composed of form and matter.[34] Aquinas’ Catholic faith, of course, did not permit him to adopt this Aristotelian view since the Catholic Church teaches and Aquinas was convinced of the existence of angels, wholly immaterial beings.

St. Thomas thought that Aristotle’s division of individual substance into form and matter was not sufficient, therefore, since it could only explain finite corporeal substances (which he referred to as “composite” substances),[35] but not finite immaterial substances (which he referred to as “simple” substances),[36] such as angels, the existence of which, Aquinas was convinced, is dictated to us even by sheer reason.[37] Now, for Aristotle, the essence of a thing was its substance (as opposed to its accidents)[38] and thus necessarily composed of matter and form. Aquinas was unable to accept this view in so far as he realized he could not extend these concepts alone—without modification—to simple substances. Hence, the Angelic Doctor insisted that the essence of a thing is not necessarily its substance. Instead, he believed that as regards composite substances, their essence is indeed their matter and form—“The term essence, used with respect to composite substances, signifies that which is composed of matter and form”[39]—, but as far as simple things are concerned, “the essence . . . is form alone.”[40] Thus, there is a fundamental distinction between the essence of a composite substance and that of a simple substance for Aquinas.

Now, Thomas realized that although a thing must have an essence, it need not necessarily exist:

Whatever is not in the concept of the essence or the quiddity comes from beyond the essence and makes a composition with the essence, because no essence can be understood without the things that are its parts. But every essence or quiddity can be understood without understanding anything about its existence: I can understand what a man is or what a phoenix is and nevertheless not know whether either has existence in reality. Therefore, it is clear that existence is something other than the essence or quiddity. . . .[41]

Thus, Thomas drew another crucial distinction, namely one between essence and existence: they are fundamentally distinct—except in God, but that shall not concern us here—, since just because something has an essence doesn’t guarantee its existence. Aquinas defined existence as that in virtue of which any finite thing has actual being:

In things composed of matter and form, neither the matter nor the form nor even being itself can be termed that which is. Yet the form can be called that by which it is, inasmuch as it is the principle of being; the whole substance itself, however, is that which is. And being itself is that by which the substance is called a being.[42]

The difference between essence and existence is very important because by insisting that not every individual finite being needs to be material, Thomas had thrown out Aristotle’s explanation of change, which, after all, was based on form and matter, reinterpreted as actuality and potentiality.

To retrieve a rational explanation of change, then, Aquinas reinterpreted essence and existence as corresponding to potentiality and actuality, just as Aristotle had done with matter and form: “Existence [stands] to the essence as act to potentiality,” [43] in the sense that the “essence is a potency with respect to the act of existence, because only those things are fully designated as substantial beings that have received existence.”[44] In other words, every finite being, whether material or not, is composed of essence and existence, and “that which actualises the essence is existence.”[45] Essence and existence are so interrelated that neither can subsist without the other: “There is no essence without existence and no existence without essence; the two are created together, and if its existence ceases, the concrete essence ceases to be.”[46]

Hence, to remedy Aristotle’s shortcomings and apply his philosophy to the Christian faith to support the latter, the Angelic Doctor modified the meaning of essence regarding substance and added the notion of existence. For Aquinas, then, the “essence of a corporeal being is the substance composed of matter and form, while the essence of an immaterial finite being is form alone. . .,”[47] while “that by which a material substance or an immaterial substance is a real being (ens) is existence (esse). . . .”[48]

With this in mind, we can now fully appreciate St. Thomas’ views on causality. With Aristotle, the saint affirmed the four classic causes: formal, material, efficient, and final. However, given Thomas’ addition of the distinction between the concepts of essence and existence, the saintly doctor was now able to add a fifth cause, the “exemplary cause,”[49] which extended Aristotle’s traditional notion of causality and made it more comprehensive, more precise, and more complete.

Partial References

[34] Aristotle, of course, distinguished between first and second substance (cf. Copleston, A History of Philosophy, Vol. I, 302-04), but to explicate this difference would exceed the scope of this paper.

[35] H. D. Gardeil, Introduction to the Philosophy of St. Thomas Aquinas, Vol. IV, Metaphysics, trans. John A. Otto (St. Louis, MO: B. Herder Book Co., 1967), 163.

[36] Ibid.

[37] Copleston, A History of Philosophy, Vol. II, 329.

[38] Stumpf, Socrates to Sartre, 88.

[39] St. Thomas Aquinas, De Ente et Essentia II, trans. Robert T. Miller, 1997; available from ; INTERNET.

[40] Ibid. IV.

[41] Ibid. V.

[42] St. Thomas Aquinas, Summa Contra Gentiles, Book II, Creation, 157; italics given.

[43] Copleston, A History of Philosophy, Vol. II, 332.

[44] Herman Reith, The Metaphysics of St. Thomas Aquinas (Milwaukee, WI: The Bruce Publishing Company, 1958), 112.

[45] Copleston, A History of Philosophy, Vol. II, 332.

[46] Ibid., 334.

[47] Copleston, A History of Philosophy, Vol. II, 332.

[48] Ibid.; italics given.

[49] de la Torre, Popular History of Philosophy, 116.