Aquinas' First Way: Unmoved Mover

Formulation

1. Some things are in motion.
2. Everything that is in motion is moved by something else.
3. An infinite regress of movers is impossible.
4. Therefore, there must be a first mover, an unmoved mover, that is not moved by anything else (God). (2, 3)

This argument can be understood as a causal argument, where it identifies that change exists, and argues that there must be a first cause of all change.

Act-Potency Distinction

Aquinas' argument relies on the distinction between act and potency.

• Act: What things are in actuality.
• Potency: What things can potentially be.

Parmenides, a pre-Socratic philosopher, argued that change is impossible. He argued that ex nihilo nihil fit, or "out of nothing, nothing comes". In his view, change would require a being to come from nothing. But because ex nihilo nihil fit, this is impossible.

Aristotle, however, argued against this. Instead of viewing change as a being coming from nothing, he viewed change as a being turning from potency to act. For example, a seed has the potential to become a tree, and when it grows into a tree, it actualizes this potential. Potency is then the middle ground between non-being and being.

Both are conceived as "modes" of being, so they are all real. To convince yourself of this, consider this: a seed can become a tree, but it also can become a dog. Clearly, one of them seems more "real" than the other, suggesting that something about being a tree is more "real" than being a dog.

However, this distinction is at best unmotivated, and at worst incoherent:

1. This distinction is incompatible with eternalism.

   The A-theory of time is the view that time is a series of moments. Up until recently, this was the dominant view of time, and is probably what most people intuitively think of when they think of time. In this view, the present is the only real moment, and the past and future are not real.

   Eternalism, or the B-theory of time, is the view that all times are equally real. Instead of thinking of time as a series of moments, we can think of time as a four-dimensional block. Modern physics, especially general and special relativity, seems to support this view.

   The problem arises when we consider Aquinas' argument in the context of eternalism. If all times are equally real, then change would not be some "actualization of potential". Instead, change is just a variance in an object along the time dimension. At all times, the object is equally real and "actual".

2. The distinction also does not solve Parmenides' problem.

   In Parmenides' view, for example, a seed becoming a tree would involve the tree not existing at time , and then existing at time . This is the problem of coming from nothing, and hence change is impossible.

   If we introduce the distinction then, the actual tree still does not exist at time ; it's not real. What's the difference?

3. The distinction presupposes pluralism about being. Aquinas' argument seems to rely on a kind of pluralism, suggesting that there are different kinds of beings. This is a controversial view, and many philosophers reject it.

Existential Inertia

The second premise, that everything in motion is moved by something else, entails a principle of existential inertia.

According to the existential inertia thesis, objects will keep existing, even without sustenance, unless something else acts on them to destroy them. Then, it means that whatever is changed is caused to change by another.

But that means that whatever isn't changed by others is unchanging. Since absences are not causes, then even in the absence of divine sustenance, an object will keep existing unless an external force acts on it to destroy it.

Validity of the Conclusion

Pay close attention to the conclusion of the argument: that there must be a first mover. This does not necessarily follow from the premises.

First, in some causal chain, there might be a first cause, and the other beings in the chain have this "causal power" derived from the first cause. For example, consider a row of dominoes. The first domino contains the causal power in respect to movement, and the other dominoes have this power derived from the first domino.

This does not mean, however, that this first cause is necessarily uncaused. This first cause might be unactualized in one respect, but perfectly able to actualize in another respect. For example, the first domino could be unactualized in respect to movement, but perfectly able to actualize in respect to heat.

Second, this is the more obvious objection: the argument does not necessarily lead to the traditional conception of God. Naturalists can accept the existence of a first cause, but they would not accept that this first cause is God. For example, they could accept a collection of physical symbols, like quarks or strings, or some quantum field or universal wavefunction, or an initial singularity. Many of these are (1) purely actual and uncaused, (2) invariant across worlds, and (3) necessary, which fulfuill the criteria that we need.