The First Way allows for the possibility that we will always be able to find some natural explanation for some fact of nature, but every explanation will involve something that falls outside the account of nature we give to explain the fact. Aristotle could give no account of the power that moved the planets in orbits and thought that they were in need of a spiritual mover; Newton then accounted for this power as gravitational action on an inertially moving body, but this left him in need of some mover to impart this inertial motion and so accounted for this with a spiritual mover. Laplace and Kant did away with Newton’s spiritual mover through the nebular hypothesis, but nebular formation ended up reducing to the laws of matter which in turn involve fine tuning – and so here we are again with some arguing that the fine tuning is caused by God while others insist we’ll find a natural explanation. The Naturalists are right that we’ll eventually discover some natural explanation of the fine tuning, but it will have its own undetermined values that will again give rise to the God vs. Natural explanation debate.
And so the First Way allows for the possibility that “Supernatural explanations will have a poor track record” and that we will continually be positing a “God in the gaps”, but it also shows us that all natural explanations will contain an undetermined element which will demand something outside what can be considered natural by the parameters of the explanation. To return to the example, Aristotle thought nature was exhausted by finite motions, and so when he located an infinite motion he had to invoke a supernatural cause; Newton posited inertial motion as a naturally infinite but this gave no account of the motion’s initiation or direction, and so when he saw the need for an initiated directed motion he had to invoke a supernatural cause; Laplace and Kant thought they could account for the direction and initiation of motion by the laws of chance starting from a formless cloud, but this turned out to presuppose fine tuning for the elements, at which point we hit the limit of present understanding. There’s no reason to expect this process ever to end. Perhaps nature is simply inexhaustible to this sort of analysis. But St. Thomas gives an argument that this inexhaustible character – or infinite fullness – must itself be an effect of pure actuality.
One benefit of seeing the First Way in this manner is that it is precisely the infinite fullness of nature that we see as dependent on divine causality. Nature is not some open-ended pipe that demands a god to blow into one end but a structure that allows for levels of analysis that become more and more simple and intelligible without end.