The first way ends by proving that an unchanging changer exists. Aquinas calls that being God, but not everyone would be convinced. One of the things true of God is that he cannot change in any way at all. By examining the unchanging changer, we can show that this changer is only unchanged if he is unchangeable.
Remember that the first way of Aquinas begins by showing that there is a being that changes something but is not changed by doing that. We could call the ability to change something else a ‘ power’. So God has the power to change something without being changed by using that power. Let’ s consider this power that God has. This power is used by God. Since God’ s use of that power involves no change, we cannot ask what caused God to use that power. If there were something that caused God to use that power, then that thing would be the unchanging changer rather than God. Furthermore, God does not have this power because of something else. If he did, then that ‘ something else’ would be causing God to have the power. That would be a change in God, and then God would not be the ‘ unchanging changer’. Therefore, God has power but that power is not used because of anything else and that power is not had because of anything else.
Compare this with the kind of powers that various things in the world have. Stones are hard, so they have the power of hardness. But this power only changes other things when the stone comes in contact with them. Therefore, stones require a cause for why the power is used. But that is not the only difference. Stones also require a reason why they are hard. Perhaps they have a particular atomic structure that makes them hard. But each stone may have a different structure, even if all of these structures gives rise to hardness. So hardness is not the same as the structure. Therefore, the cause of hardness is the structure of the stone. Since neither of these is true of God, God is very different from a stone.
If God’ s power to change something were changeable, then there would either be a reason for the power being used or a reason for having that power. He power could either be changeable by being gained or lost, or by being used. If it can be changed by being gained or lost then there is a reason for that change. If it is being used, then it is being used by something else. Since God does not change when using this power, neither of these is true, as proven above. Therefore, God’ s power to change something is not changeable. So the unchanging changer is also unchangeable. Theologians refer to this truth as the immutability of God.< Right now, we have only proven that there is an unchangeable changer, who is the first cause of at least one change in the world. This change could be the change in the weather or something else. We have no yet proven that there is only one changer, or even that this changer is the cause of all change in the world. That will be proven later.