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.

Aquinas’ five ways are often criticized because, unless further argumentation is supplied, their conclusion is not obviously identical to the God of classical theism. Such criticism overlooks the semantic and hermeneutic functions of natural theology. By fixing the referent for the word ‘God’, the five ways allow the philosopher to provide an intelligible account of divinity. This knowledge of the cause through its effects follows the systematic structure of the triplex via (causation, negation, and eminence), a program that guides Aquinas’ account of divine attributes all the way through Summa Theologiae I qq.2-26 and Summa Contra Gentiles I. By following this rational itinerary the demonstrative power of the ways can be better assessed, looking at them not as an apologetic exercise, but as a first step in the seek for a deeper understanding of the divine source of Creation.

Aquinas’ fifth way for demonstrating the existence of God shows a confidence in its argument that is not generally shared by the contemporary reader. Natural entities that lack awareness (or perhaps self-awareness) act in an end-directed manner; this implies a fundamental relation to goals they do not consciously choose; this in turn points to the existence of a governing mind. The article tries to make sense of this argument against the background of philosophical naturalism that is assumed by much contemporary thought. It addresses the question of what kinds of end-directed activity fall under the scope of the argument, and why Aquinas thinks that these imply a governing mind. The article notes the unusual structure of the argument, suggesting that it should be understood (as should all of the five ways) as an attempt to wake the reader up to something so fundamental that it is usually overlooked. The main difficulty for the contemporary reader is to recover an Aristotelian sense of what it means for something to exist. If readers can achieve this, they may come to appreciate why an argument that seems enormously controversial in the present day seemed reasonably obvious to Aquinas.