Let’s see if we can get the ball rolling a bit more. God, according to classical theism, is thought of as Pure Act, which means He is being-itself-subsisting. Now, what does this mean?
Well, as it has been discussed on other threads, an attribute most existing entities have is dynamism. Things change, e.g. they are in motion from potentiality to actuality (act). An acorn, for example, is in actuality just an acorn, but in potentiality it is an oak tree. Now, nothing actualizes itself, which means that the acorn needs some thing(s) that have actuality to actualize its (the acorn’s) potentiality. For example, it needs soil, water, and sunlight, among other things.
Applied to cosmic motion, or change, we might ask: what is the cause of this motion? The gears of a watch will not move if there is no spring, even if there are infinitely-many gears. The boxcars of a train will not move if there is no engine, even if there are infinitely-many boxcars. By analogy, if the cosmos has no First Mover, then none of the heavenly bodies (stars, planets, moons, etc.) will be in motion, which is patently false. Therefore, a First Mover is needed to cause the motion of everything else.
Of course, if the First Mover were in potentiality at all, then its motion would need to be actualized by another, as we saw already. Since this is impossible, it follows that the First Mover is Pure Act. By way of summary:
- Evident to the senses is motion. (Premise)
- Everything in motion is moved by another. (Premise)
- If there is no First Mover, then nothing will be in motion. (Premise)
- Therefore, a First Mover exists. (From 1 and 3)
(2) is included because it is often asked: if everything has a mover, then what moved the First Mover? However, such a question misinterprets the causal premise. It is not that everything is moved by another, but that
everything in motion is moved by another. So, the First Mover is Pure Act, and no transition from potentiality to actuality can exist apart from Pure Act.
The First Mover, or Pure Act (God), is therefore the ground (cause) of all being and becoming.