How do you understand St. Thomas’ first “proof” (Prime Mover) for the existence of God (I, Q.2, Art.3)?
Let’s start with Aristotle’s definition of motion, which is basically that “”Motion is the actualizing of what exists in potency insofar as it is in potency.” Potency means changeable. This leads us to understand motion as “the actualizing of the moveable (changeable) insofar as it is moveable (changeable).”
So, God is pure act. Therefore, there is no potency in Him, and He cannot move (He cannot be the object moved) but He can move [verb] others as cause.
All contingent things are obviously not pure act, but at a minimum have the potency to exist or not to exist. So this potential to exist must be brought about by something other than themselves, since they do not explain their own existence. This “movement” from potentiality is from God.
Of course, this will include not only things that involve local motion, which are the material things we study in physics, but also the “motion” of moving from potency to act even in non-material beings, such as angels, etc.
In the end, Thomas’ argument is one that says that nothing that does not explain its own being can be the cause of its existence (almost a tautology), and so a pure act, a necessary being, must exist, an uncaused cause and unmoved mover.
So at it’s core, motion is about going from non existence to existence?
At its core, motion is moving from potency to act. The way we see this as humans is generally in the physical world, which is local motion, physical motion of material things. But also, say, in the realm of mind, moving from not knowing something to knowing it (by being taught, for instance). Motion always involves potency. God, having no potency, cannot receive motion, cannot receive act.
So it’s potency to act in the broadest sense, meaning it covers every type of potency to act?
Yes. Going from non existence to existence, in the sense of creation, however, is a very different thing than going “from here to there” or going “from black to white,” etc. it’s not like God took some thing called “nothing” which had potency and actualized it. He rather made something to exist from nothing-at-all.
Much harder to perfectly define, but the point still remains: for anything to exist that could possibly not exist, a necessary being (pure act) must be the cause.
This stuff is so important in understanding Thomas’ teachings. For example, many problems arise from misunderstanding things of God as if they were movements within the world. A couple examples should demonstrate:
1. When you read Thomas treatment of the processions in the Trinity, you will see that the heresies of Sabellianism and Arianism both treated the procession of the Son from the Father as if it was a movement from within the world, and this is why they could not understand one God and three Persons.
In the first objection in Q.27, it is said “It would seem that there cannot be any procession in God. For procession signifies outward movement. But in God there is nothing mobile, nor anything extraneous. Therefore neither is there procession in God.
The reply to this objection is “This objection comes from the idea of procession in the sense of local motion, or of an action tending to external matter, or to an exterior effect; which kind of procession does not exist in God, as we have explained.”
2. In creation, so many of the debates between evolutionists and intelligent design adherents is based on a false understanding of creation as something that happens “within” the world. Thomas’ answer (and I believe the correct one) sees that it transcends this. Here is an excellent video that demonstrates the point well:
and part 2:
The point is, understanding act and potency, existence and essence, etc, is crucial for understanding so much about what Thomas says, and thus, about God and the world.
One last point can be made here:
There is a difference between cause and effect and sufficient reason. People sometimes miss this. For instance, God is not the cause of Himself; He has no cause. He is, however, the explanation, the sufficient reason, for His own existence. Some do not understand the difference here, and it leads to other problems later.
Thus, people will say that, by saying everything needs a cause, we must give a cause for God. But Thomas, for one, does not say that all things need a cause, but rather, all contingent things need a cause. All caused things need a cause, and all contingent things are caused things. God, who’s essence is to exist, is not contingent, and thus does not need a cause.