Argument from Change: 'Turtles all the way down. Something from potential. First cause.'

The 'turtles all the way down' quip works against a *popular* misreading of the cosmological argument as a chain of events stretching back in *time* (a 'per accidens' series, where each cause need only have existed, not currently act, to produce its effect — e.g., grandfather begets father begets son). Aquinas's actual First Way concerns a 'per se' (hierarchically, simultaneously ordered) causal series, where each member's causal power is not its own but is instrumentally derivative from what is actualizing it *right now* (Edward Feser, *Aquinas: A Beginner's Guide*; *Scholastic Metaphysics*, ch. 1-2). The classic illustration is a hand moving a stick moving a stone: the stick has no motive power of its own; it moves the stone only insofar as, and for as long as, the hand is actively moving it. In such a series, an infinite regress of purely instrumental movers explains nothing, because none of the intermediate members contributes any causal power of its own — remove the first, underived source of actuality, and the whole simultaneous series collapses, however many links it has. This is a structural, not merely numerical, argument: 'turtles all the way down' fails specifically because turtles-with-no-ground-turtle never actually supply any standing on which to stand. The regress objection has bite against a temporal-chain reading; it has no bite against the hierarchical reading, which is what Aquinas is actually arguing (Summa Theologiae I, q.2, a.3).

The argument concludes to a purely actual actualizer — something that need not (indeed cannot) be actualized by anything further, since it has no unactualized potency left to be actualized. Aquinas identifies this with God, and unpacks further divine attributes (simplicity, immutability, eternity, oneness) as entailments of pure actuality in subsequent questions of the Summa.