In his dialogue On Free Choice of the Will (Macmillan, 1964), St. Augustine (354-430 C.E.) argues that our minds can know truths that are eternal. For Augustine, something is eternal if it exists in a timeless, unchanging state. So eternal truths are unchanging and are not in, or influenced by, time. For example, consider the following three propositions:
- A cannot be both B and not B at the same time and the same respect (the law of non-contradiction)
- Either A or not A (the law of excluded middle)
Augustine would claim that these propositions are eternal truths. This means their truth never changes and is not dependent on any event in the past, present, or future. These truths hold for everyone, everywhere, at all times. To be sure, changing beings can think these truths in time and they can be represented in various languages and sentences that change. But the truths themselves never change.
Now ask yourself: could human minds produce eternal truths? Well, if eternal truth exists then it is unchanging. But human minds are always changing. So eternal truth cannot be a product of our minds. Augustine writes: “If truth were equal to our minds, it would be subject to change. Our minds sometimes see more and sometimes less, and because of this we acknowledge that they are mutable. Truth, remaining in itself, does not gain anything when we see it, or lose anything when we do not see it” (67). We could also add that there was a time when humans did not exist. So eternal truths, as always existing, cannot possibility be the product of our minds. But if human minds didn’t produce eternal truth then what did? Well, nothing produces eternal truth because, being eternal, it always exists. But perhaps some mind, rather than producing eternal truths, always has eternal truths in mind. Such a mind would have to be eternal itself if the truths it possesses are eternal. Augustine thinks this line of reasoning strongly suggests an eternal being exists who sustains eternal truths. He writes:
“You granted, moreover, that if I showed you something higher than our minds, you would admit, assuming that nothing existed which was still higher, that God exists. I accepted your condition and said that it was enough to show this. For if there is something more excellent than truth, this is God. If there is not, then truth itself is God. Whether or not truth is God, you cannot deny that God exists, and this was the question with which we agreed to deal.” (71)
So the argument of the dialogue is as follows:
Premise 1: If there is something unchanging and not created by human minds, then God exists.
Premise 2: Truth is unchanging and not created by human minds.
Therefore, God exists.