Thomas Aquinas delivers a proof for the existence of God in which he first shows that there is something that necessarily exists, and then goes on to show that among the things that necessarily exist there must be something that derives its own necessity, and this “all men speak of as God.” The main objection to Aquinas’ argument is that it contains a Parmenidean fallacy of substantial change. To counter this objection, we must salvage Aquinas’ conclusion by reconciling parts of his argument with a natural philosophy of qualitative change.

Aquinas’ argument is quite succinct. We find things in nature whose non-existence is possible, for all around us we see things generated and corrupted. A thing’s non-existence is possible only if at some point it does not exist, so if every thing’s non-existence were possible there would be a point at which nothing existed. If there ever were such a point of universal non-existence, nothing would exist now, for that which exists comes only from that which existed previously. It is absurd to think that we are living in an era of universal non-existence, so there has never been such a point in time, and therefore it is not the case that every thing’s non-existence is possible. In other words, some thing’s non-existence is impossible; something necessarily exists. Among the things that necessarily exist there must be something whose existence does not rely upon another necessarily existing thing in order to avoid an infinite regress. Therefore, something necessarily exists and derives its own necessity, and this “all men speak of as God.”

Parmenides helps us discover the sly error in this argument. If we comply with Parmenides’ legacy by denying substantial change and instead resorting to a natural philosophy of qualitative change, we see that Aquinas’ first premise, that we find things in nature whose non-existence is possible, is false. We have no experience of things possibly existing, for when we witness the generation and corruption of things, we understand this to be qualitative, not substantial change. To assert that all change is qualitative invalidates Aquinas’ claims by showing that upon generation and corruption, things do not go in or out of existence. If no thing can possibly not exist, then every thing has a trivially necessary existence, and Aquinas’ argument is unappealing to all but pantheists.

We respond to this objection by revisiting the argument’s final premises and finding them consistent with a natural philosophy of qualitative change. The final premises hold that in order to avoid an infinite regress there must be some necessarily existing thing whose existence does not rely upon any other necessarily existing thing, and therefore derives its own necessity. These claims do not rely on notions of possible existence so they avoid the charge of the objection, and they are sufficient to establish Aquinas’ conclusion under the condition that some things exist necessarily, a claim following from a natural philosophy of qualitative change. The remainder of the argument needs some retooling to be made sound, but at least we’ve shown that the argument’s conclusion can be made consistent with its greatest objection.