An Appraisal of Aquinas’ First Way: Introduction (Part 1)


1. Introduction

The Five Ways of demonstrating God’s existence were given systematic treatment by arguably the most renowned and respected Christian philosopher of all time, St. Thomas Aquinas. Born in Italy in the thirteenth century, Aquinas sought to synthesize an Aristotelian framework with a Christian worldview. One integral aspect of this synthesis was Aquinas’ use of Aristotelian metaphysical notions in his arguments for God’s existence — notions such as actuality, potentiality, essence, efficient causation, natural teleology, and so on. In fact, Aquinas goes so far as to characterize his First Way as the more “manifest” way, indicating the centrality of Aristotelianism in his thought.1

Because of its evident centrality to Aquinas’ thought and Aquinas’ far-reaching influence, I shall offer fresh paths in analyzing and evaluating Aquinas’ First Way. As a cosmological argument, it starts from a broad feature of the world (viz. change) and proceeds by means of a causal principle to argue that there must be some fundamental or ultimate explanation of the feature in question. In what follows, I will do a number of things. First, I will provide a brief characterization of Aquinas’ argument by means of textual exegesis, followed by a formalization of the argument in Part 2. After formalization, I will critically evaluate the argument by means of a discussion and evaluation of objections, with the ultimate conclusion that Aquinas’ argument does not succeed in establishing the classical theistic God. Along the way, however, I will develop important maneuvers on behalf of Aquinas that serve to enhance the dialectic and rebut certain lines of argument drawn against Aquinas. Finally, I will present a novel argument from change along broadly Thomistic lines that aims to avoid the pitfalls of Aquinas’ argument.

2. Textual Exegesis and Explanation

Aquinas begins by noting the evident fact that things undergo various changes. Change, though, is just the actualization of a potential: “For motion [change] is nothing else than the reduction of something from potentiality to actuality.”2 In turn, Aquinas affirms a causal principle, stating that “whatever is in motion is put in motion by another.”3 In other words, whatever reduces from potentiality to actuality is changed (i.e. actualized) by something else in a state of actuality. In defense of this, Aquinas points out that nothing can be both actually X and potentially X in the same respect and at the same time, for then it would be both actually X and not actually X, which is absurd. But given that change is the actualization of a potential, and given that only actually existent things can cause/actualize changes, it follows that no mere potential can cause or actualize itself, for then it would have to be actually existent in order to have the actual causal power to do so (contrary to our supposition that it was merely potential). As a result, Aquinas emphasizes: “Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it. Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect… It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself.”4

It is worth noting that, thus far, Aquinas’ defense of the causal principle is parallel to an argument for the metaphysical impossibility of self-causation. Nothing can cause itself to exist, for then it would have to exist prior to its own existence in order to bring its own existence about. It would therefore be both prior to and posterior to its own existence, which is absurd. Hence, nothing can cause itself to exist. Similarly, Aquinas argues that no mere potential can change itself, for it would have to be actually existent in order to have the actual causal power to actualize anything at all — in which case it would not be merely potential (contra our original supposition). Hence, nothing can change itself. In turn, Aquinas takes this to show that “whatever is in motion must be put in motion by another.”5

Notice, though, that we now have chains of change within the world. Things change. But that means, per the causal principle, that they are changed by other things. But if those other things are themselves changing, they require yet further actualizers. This series, however, cannot proceed to infinity, “because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand.”6 In other words, if there is no first member in the series of changes, then there would be no other changing members. Clearly, though, there are other changing members, since the world of our experience is replete with changing things. “Therefore,” Aquinas reasons, “it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.”7

In summary, Aquinas argues as follows. Things change. But change, as the actualization of potential, requires a changer. But the series of changers cannot be infinitely long, meaning it must terminate in a first member which changes but is not itself changed (for if it were changed, then it would not be the first member since that which changes requires some further changer). This unmoved mover or unactualized actualizer, in turn, is what Aquinas understands to be God. Hence, God exists.

In the next post, we will formalize Aquinas’ argument, with subsequent posts analyzing and evaluating various objections leveled against the argument.

Author: Joe


  1. Aquinas, “Summa Theologiae,” ST I q2,a3.
  2. Ibid.
  3. Ibid.
  4. Ibid.
  5. Ibid.
  6. Ibid.
  7. Ibid.