Natural theology and natural law › Change: potentiality and actuality

The first Way begins from motion, understood through the transition from potentiality to actuality. Potentiality is a capacity to be otherwise; actuality is the relevant condition being realized. Aquinas's example concerns fire heating wood. The wood can become hot, and something already actual in the relevant respect brings about the change (I q.2 a.3).

The qualification concerning respect matters. Aquinas does not say that a thing cannot be actual in one way and potential in another. Wood can already be warm while capable of becoming warmer. His principle denies that the same thing is actual and merely potential in precisely the same respect at the same time.

He then argues that whatever is moved requires a mover, and that a series of movers cannot proceed without a first source. Otherwise, on his account, the intermediate movers would not exercise the derivative activity through which change occurs. He concludes that there must be a first mover that is not itself moved.

This is not simply the claim that every object needs an initial shove at some date in the distant past. The text's explanatory framework concerns the dependence of movers, including an example of a staff moved by a hand. Reading the Way solely as a claim about the earliest event misses the role of that dependence.

A critic can question whether every instance of change requires the actuality Aquinas specifies, or whether the regress principle excludes the relevant infinite series. Naming these commitments makes the disagreement more precise than merely announcing that motion exists. The observation initiates the argument; it does not by itself establish the first mover.