“Does Aquinas’s Fifth Way remain defensible?”

Aquinas’s Fifth Way to demonstrate the existence of God is based upon a particular notion of teleology. This notion differs from others commonly associated with teleological arguments. In the Fifth Way, teleology is not thought as purpose or harmony, but rather as the directedness that beings display in their activities (what I call immanent teleology). Aquinas thought his notion of teleology would apply indistinctively to all natural beings and that the existence of such teleology in unintelligent beings would require the existence of a superior intelligent being, namely God. Some critics of Aquinas’s argument claim that the development of biology and science in general since Aquinas have refuted his teleological premise – especially when one thinks about how evolutionary accounts of biology seem to explain away any need for teleology in order to make sense of biological phenomena. I think these critics are wrong and that the Fifth Way still holds as a defensible argument for the existence of God. In this scenario, I believe the defender of the Fifth Way has two options: first, to offer accounts of irreducible immanent teleology in the biological realm; second, to narrow Aquinas’s premise, claiming simply that at some basic level of reality there has to be irreducible immanent teleology. I also argue that the second option does not disfigure Aquinas’s metaphysical assumptions and is more defensible than the first. Finally, I offer replies to common objections to the Fifth Way and try to integrate my explanations in the bigger picture of the argument, showing how I understand it still remains defensible.