In the last two posts, we looked at the general idea that if something is composite, it must be held together by something else. But it's still very abstract. Let's look at a more concrete example.
Remember the stone that can't move itself? Keep that in mind at all times.
If something changes, it goes from one state to another state. That is, it goes from an actual state to a possible state, and the possible becomes actual.
But something cannot be both actually one way and possibly that same way at the same time. For example, liquid water cannot be actually ice and possibly ice at the same time. This is a logical contradiction.
Therefore, something cannot be bother changer and changed. That is to say, it cannot change itself. Remember the stone? The stone cannot move itself.
So whatever is changing cannot change itself, and something else must be changing it. In the case of the stone, we see that an actual stick is changing the stone's location. In the case of water freezing into ice in an ice cube tray, we see that actual cold air is changing it into ice.
But now the same thing applies again, to the stick and the cold air.
The stick is not moving the stone, because sticks can't move themselves either. So we still don't know what is moving the stone and we have to keep going.
Somewhere in this chain, there must be something that can push without being pushed, or cause change without being changed. An unpushed pusher, or an unchanging changer.
Something unchangeable must be purely actual, with no possible future states. And we are thus led again to the existence of God.