The Review of Metaphysics

The first of Aquinas's famous five proofs is often thought to fail due to its reliance upon the impossibility of an infinite regress of movers or outdated Aristotelian physics. In Nature and Nature's God Daniel Shields takes on the impressive task of replying to these and other standard objections. The book is divided into two parts. The first section advocates for a novel, modest understanding of the first proof, corrects common misinterpretations of it, and integrates the first proof with other arguments for God's existence. The second part defends the relevance of the motion proof in the context of modern physics.

The first three chapters are dedicated to interpreting the first proof itself. Shields clarifies that "Aquinas's ultimate goal is not to establish that God must exist in order to initiate order at the beginning of the universe, but rather that God must exist in order to explain motion now." He is at pains to emphasize the first proof is an argument from change; it alone does not have the resources to show that God is pure act, as some interpreters have thought. In addition, he argues that Aquinas's theory of motion is compatible with the view that a mover need not be in contact with a projectile for the entirety of its motion.

Shields explains Aquinas's key premise that "what is in motion can only be moved by what is in act." To change from rest into motion something must first have the potential to be moved. However, "the same thing cannot be in potency and act in the relevant respects at the same time" because the two states are mutually exclusive. But since what is merely in potency can have no causal impact on the world, it follows that "for every motion there is a first mover that is already in act." Next, he recalls the distinction between an essentially and accidentally ordered series of causes.

An accidentally ordered series can proceed infinitely backward in time, whereas an essentially ordered one cannot. An infinite essentially ordered series is impossible because the members of such a series do not in themselves have the power to produce their effect. For example, "a boxcar cannot account for the motion of the caboose because a boxcar is only a moved mover. . . . The same would be true of an infinite number of them coupled together." If there is an infinite series of causes none of [End Page 555] which in themselves has the power to move, then such a causal series alone could never explain some instance of motion. Thus, an essentially ordered series of causes requires a first mover that in itself has the power to move. Notably, Shields thinks Aquinas has no intention of ruling out purely natural entities as first movers in the first way; rather, according to Shields, Aquinas wants to establish only the need for some first mover(s), but not necessarily God.

The further task of proving the God's existence is left to other of Aquinas's arguments, considered in chapters 4 and 5, which rely on the principles defended in the first proof. Shields notes that motion in the universe either has a beginning or is perpetual; in both cases, God must exist to create or sustain it. God is required to sustain a perpetual universe because the constituents of the universe are corruptible. Given that in an infinite amount of time all possibilities will be realized, it follows there must be some point in the infinite past where all possible beings are simultaneously corrupted. As Shields points out, "in that case, there would have been nothing at all and there would still be nothing, contrary to fact." On the other hand, God is required to explain a universe or causal series with a beginning because without God, there would be nothing...