Aquinas’s Fifth Way argues for God’s existence from the perception of goal-directed activity in nature. Its details are difficult to understand. This study interprets the premises and offers background reasoning for them, which Aquinas develops elsewhere in his writings. A major focus is clarifying the scope of finality the Fifth Way invokes. The argument leaves unspecified the kinds of purposive activity in nature Aquinas has in mind. Thus, the discussion first treats types of purposive activity in nature Aquinas recognizes. It then looks at the two reasons the argument gives for final causes in nature. Things tend to act in regular ways and tend toward what is ‘best’. Attention then turns to the key premise that goal-directed activity in nonrational beings requires direction by something with intelligence. A final section of the article explores why Aquinas seems to look to a single source of finality in nature and why, in the conclusion, he claims that we call this God. Thus, Aquinas’s larger views on finality in nature shed light on his intents in the Fifth Way.

When academically inclined atheists critique arguments for the existence of God, they commonly target arguments of Intelligent Design as proposed by Paley, Dembski, and Behe. In so doing, it is not uncommon for them to include within the scope of their criticisms Aquinas's fifth proof for the existence of God – the proof from final causality. In this essay, I shall argue that there are very significant differences between the Fifth Way and the more modern arguments of Intelligent Design which means that any critique offered in regard to the latter normally leaves the former unscathed. Moreover, I shall also argue that the Intelligent Design approach concedes some of the erroneous premises of the atheists’ own arguments and that these are in no way conceded by Aquinas in the Fifth Way or elsewhere.