You may not be familiar with St. Anselm, but he was a famous theologian in Aquinas’ day. He’s best remembered for his “ontological argument.” Besides Aquinas’ “Five Ways,” it’s probably the most famous argument for God’s existence in Catholic tradition.
And Aquinas rejected it.
“What!” you might say. “Why would he do that?”
Good question! Buckle up, put on your philosophy hat, and let’s dive in.
How the argument goes
St. Anselm’s ontological argument goes something like this: God is defined as “that than which nothing greater can be conceived.” In other words, God lacks no conceivable perfection.
Now suppose you accept this definition of God—but you say He doesn’t exist in real life, He’s only a conception in the mind. God would have all the same attributes as He would in real life, except for one: to exist objectively, outside the mind. But it’s more perfect to exist objectively than to exist only in someone’s mind. So the concept of God in the mind lacks this perfection.
Yet God, by definition, lacks no conceivable perfection. Therefore, God must have a real existence outside the mind.
Why Aquinas rejected this argument
The ontological argument rests on the idea that we can know God’s essence in itself: specifically that God, by his essence, necessarily exists.
Aquinas says no, that’s not self-evident to us — at least not here on Earth. In Heaven, we’ll see God as He is, and it will be self-evident that God is His own existence. But in the meantime, the proposition that “God is his own existence” is not self-evident to us. We can’t just start with that definition of God. We have to logically arrive at it, as a conclusion.
This doesn’t mean St. Anselm’s argument was bad. Aquinas had a great respect for the theologians and philosophers who came before him. He inherited their traditions and built his own work on their foundations. But he was intellectually honest enough to critique them.
And that’s a great example for us, if we want to learn from great thinkers, yet be good critical thinkers too.