We’re blogging through St. Thomas Aquinas’ Compendium Theologiae, sometimes called his Shorter Summa. Find the previous posts here.
As we saw in Chapter 4, God, as God, is utterly unchanging. (Jesus, God Incarnate, is a man and so is clearly subject to change: he is conceived, born, grows, dies, and is raised. But we’re not nearly there yet.) And since God is unchanging, then, Thomas argues that God must be eternal.
The further conclusion is evident that God is eternal. For everything that begins to be or that ceases to be, is affected in this way through motion or change. But we have just shown that God is absolutely immobile. Consequently He is eternal.
There’s a footnote in Aquinas’ Shorter Summa that suggests that this chapter was perhaps left in the finished work inadvertently, as Chapters 6 and 7 treat of the subject in considerably more detail. Nevertheless, I like this one, because I can understand it. Generation and corruption, coming to be and ceasing to be, are kinds of change. Consequently, the first mover, being unchanging, never comes to be or ceases to be. And something that never comes to be nor ceases to be must either never exist or always exist. But the first mover must exist. Thus, the first mover, God, must always exist, that is, must be eternal.There’s more to it than that, though. To be eternal is to be outside of time: there is a real difference between being eternal and simply living forever, as we’ll see when we get to Chapter 7.
But how is God outside of time? A thing that is utterly unchanging is certainly immune to the passage of time, as one might say: time has no effect on it, does not wear it down, leaves no trace. But a thing might be unchanging and still exist within time.
But let’s look a little deeper. By the term “God”, at this point in Thomas’ argument, we mean the “Unmoved Mover,” that which is unchanged but ultimately responsible for all change in others. Now, either Time is identical to the Unmoved Mover, or is caused by it.
According to Aristotle, Time is the measurement of change; it would seem that Time arises precisely because there are beings that come to be, pass away, and change in other ways. Time is an effect of the reality of change, not the cause of it; and hence it can’t be God. Rather, it is caused by God, along with everything else, and so God is necessarily outside of Time, not bound by it, as an author is outside of the book he writes.
But these are deep waters. St. Augustine famously observed in his Confessions that he understood what Time was perfectly well, until he actually sat down and thought about it, and then he was all at sea. Let’s move on.