PS: I am NOT an expert in philosophy, so please, if I had made an error in reasoning please inform me.
What exactly are the differences between Aquinas’ first 3 ways for the existence of God. It seems to me that fundamentally speaking, they all basically say that “Things exist but they don’t need to exist, so there must be something that exists by necessity, which is what we call God”
Am I mistaken on something here?
Im not an expert either. Here is my take on Aquinas’ 5 ways. Some of the arguments are similar and argue for a first cause (i.e. first cause of change - unmoved mover, first cause of existence) . Notice, Aquinas does not yet show that the first cause is God. Instead he simply says that we think of it as God or it is what people mean by God. After the 5 arguments he goes on to demonstrate why the first cause he spoke of is God. So first he demonstrates there is a first cause. Then later he demonstrates this first cause must be God.
This sounds like the contingency argument. Everything is contingent except the first cause which must be eternally uncaused.
An analogy to this is the coffee cup example. A cup on the table is being held up by the table. And the table is being held up by the floor. And the floor is being held up by the building. And the building is being held up by the earth. In this example we can think of the earth as the first cause and everything else is contingent upon the earth for it’s own causal powers in the chain of causes. Each cause derives its causal power from the earth. If there was no foundation nothing else could have the power to hold up the cup. Thus in this example there must be a first cause. Not first in time since all causes must be active at the same time. But first in the causal sequence.
This is the kind of first cause Aquinas has in mind for the cause of existence. A first cause that must be present at all times for anything else to exist at any time. For instance the cause of my existence at any moment could be a series of causes that must terminate at a first cause. One’s bodily structures, cells, atoms, electrons, quarks, etc, exist in a line of causes that all derive their causal powers ultimately from the first cause. This first cause must be uncaused and in existence always or eternal. In fact it must be existence itself. That is its essence must be to exist.
The line of argument is similar for each, but the starting premises are different.
The argument from motion is based on the premise that anything reduced from potency to act is actualized by another. It has its basis in the idea of change and the distinction between act and potency. He demonstrates that there must be a Prime Mover, something which just is actual and has not been reduced from potency to act by something else.
The argument from causation is founded in the idea of efficient causes and that no thing can be an efficient cause of itself. In such a way he demonstrates that there must be a First Cause, something which itself has no prior efficient cause.
The argument from contingency, that things could possibly be or possibly not be, and so demonstrates that there must be a Necessary Being, which exists by its own intrinsic necessity and not one derived from anything else.
So from three evident starting premises of change, efficient causality, and contingency we can see that God must exist.
Note that the first article of the Summa Theologica doesn’t rule out multiple Prime Movers, First Causes, or Necessary Beings in itself, or establish that these end up all being the same thing, but later arguments do so. He also elsewhere establishes the Prime Mover’s/First Cause’s/Necessary Being’s immutability, eternity, oneness, goodness, perfection, will, omniscience, omnipotence, and so on.