Medieval Philosophy

by Peter S. Adamson

exist. A typical Augustinian argument to this effect was that God is truth, and it is self-defeating to deny that there is truth: to do so you’d have to say that it is true that there is no truth (Q2 a1 obj 3). This sort of reasoning had been used by Anselm, who was, of course, responsible for an even more famous attempt to show that God’s existence simply cannot be denied. This was his ontological argument, which Aquinas interprets as trying to establish that God’s existence is self-evident (Q2 a1 obj 2). For Aquinas the argument fails, because it concludes from what must be the case in our minds to what must be the case in external reality. Though Aquinas does put his finger on a feature of the argument that makes people uncomfortable, I don’t think it is a good objection. After all, the whole point of the ontological argument is to move from our idea of God to God’s real existence. Aquinas simply rules this out as an illegal kind of inference. This amounts to stipulating that the ontological argument doesn’t work rather than pinpointing where exactly the flaw in Anselm’s reasoning might be. If God’s existence is not self-evident, how should it be established? On the basis of sense perception, of course, since it is on the basis of the visible created world that the invisible must be known.7 Each of the five ways duly takes its departure from a feature of the world of sense experience. Take the first of the ways. It argues that every motion depends on some cause which mo...