CT 6: Necessity of God’s Existence

In Chapter 6 of the Compendium Theologiae, Thomas shows that God necessarily exists:

The same line of reasoning clearly shows that God necessarily exists. For everything that has the possibility of being and of not being, is mutable. But God is absolutely immutable, as has been demonstrated. Therefore it is impossible for God to be and not to be. But anything that exists in such a way that it is impossible for it not to exist, is necessarily Being itself, ipsum esse. Necessary existence, and impossibility of nonexistence, mean one and the same thing. Therefore God must necessarily exist.

Now, I’ve got a problem here. I can see that if a being is immutable, it must either exist eternally, or never exist at all. Since it cannot change, it cannot begin to exist. But Thomas is saying that only the former possibility works: that because God is immutable, it’s impossible for Him not to exist. I don’t see why that should be.

Granted, we exist; I don’t hold with folks like Descartes who go around doubting what’s plainly in front of them. And if we exist, that implies a first mover, as we settled in CT 3, and so God exists. But why couldn’t He not have existed, and everything else along with Him?

There’s clearly a subtlety here that I’m missing.

Moreover, everything that has a possibility of being and of not being, needs something else to make it be, for, as far as it itself is concerned, it is indifferent with regard to either alternative. But that which causes another thing to be, is prior to that thing. Hence something exists prior to that which has the possibility of being and of not being. However, nothing is prior to God. Therefore it is impossible for Him to be and not to be; of necessity, He must be.

This seems to me to be just a restatement of the “first mover” argument. If all Thomas is saying is that because we exist, God must necessarily exist, I’m fine with that. But he seems to be making a stronger statement.

And since there are some necessary things that have a cause of their necessity, a cause that must be prior to them, God, who is the first of all, has no cause of His own necessity. Therefore it is necessary for God to be through Himself.

And here there are clearly depths I haven’t plumbed: “It is necessary for God to be through Himself”. I gather what this means is that you and I depend on on God for our being; but that God simply is. If God “is necessarily Being itself,” as Thomas says above, then I suppose it makes sense that He must exist; how can Being itself not be? But I still don’t see the force of Thomas’ argument.

I will note that the idea of God’s necessary existence, an idea that I believe goes back to Aristotle, dovetails neatly with God’s name for Himself in Genesis: “I AM”.

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