Argument from Motion

Aquinas's First Way begins from the observation that things change: they move, grow, heat up, cool down, come to know things they did not know before. Each such change is analyzed as the actualization of a potential that was previously merely possible. A cold object has the potential to become hot; when it does become hot, that potential is actualized.

The argument holds that nothing can actualize its own potential in the same respect at the same time, since to do so it would need to already be actual in that respect, contradicting the premise that it is merely potential. Hence whatever changes is changed by something else already actual. Aquinas then argues this cannot regress infinitely and concludes there must be a first unmoved mover, purely actual, which all call God.

The argument descends directly from Aristotle's Physics Book VIII and Metaphysics Book XII, where the unmoved mover functions to explain the eternity of celestial motion within Aristotle's cosmology. Aquinas adapted this framework, stripping away Aristotle's specific astronomical commitments while retaining the metaphysical core: change requires an already-actual actualizer.

Medieval Islamic philosophers, particularly Avicenna and Averroes, transmitted and refined Aristotelian causal metaphysics before Aquinas, and the First Way reflects this synthesis of Aristotelian physics with monotheistic theology.

1. Some things are in motion, that is, undergoing actualization of potency. 2. Whatever is moved is moved by another already actual thing, since nothing can actualize its own potency in the same respect. 3. If that mover is itself in motion, it too requires a prior actual mover. 4. This regress of essentially ordered movers cannot proceed to infinity, since an infinite regress of purely derivative movers would never ground the motion presently observed. 5. Therefore there must be a first unmoved mover, itself pure actuality, which all call God.

The argument's logical structure closely parallels the Second Way (Argument from Causation), differing mainly in whether it analyzes change in general (First Way) or efficient causal dependency (Second Way).

The first premise is empirically secure: change is ubiquitous and undeniable in ordinary experience and confirmed by every branch of natural science. The second premise, that whatever changes is changed by another, rests on the plausible principle that a potency cannot actualize itself, since actualizing requires already possessing the relevant actuality.

The regress premise requires distinguishing per se (essentially) ordered causal series, where each member's power is entirely derivative and simultaneous, from per accidens (accidentally) ordered series extended in time, such as a line of ancestors. Only the former, Aquinas argues, cannot regress infinitely, since a purely derivative chain never actually possesses the power it transmits.

The most common modern objection charges the argument with equivocating on 'motion': Newton's first law states that bodies in uniform motion continue in that state without needing a mover, seemingly falsifying premise two for ordinary local motion. A second objection, from Anthony Kenny and others, questions whether Aristotelian act/potency concepts have any genuine application to modern physics, which describes change in terms of mathematical laws and conserved quantities rather than actualized potencies.

A third objection observes that even granting a first unmoved mover, the argument does not establish that this mover is one, personal, or identical to any specific deity, since in principle multiple or impersonal first actualizers might be conceivable.

In reply to the equivocation objection, Thomists point out that inertial motion, as constant velocity, is not itself an actualization requiring further explanation in the relevant sense, while any change in velocity (acceleration) or any qualitative or substantial change remains subject to the premise; the argument's scope is narrower than 'all motion' but still covers pervasive phenomena.

In reply to Kenny, Thomists like Feser argue the act/potency distinction is a metaphysical claim about the structure of change as such, compatible with, and not falsified by, any particular physical theory, since physics describes correlations and laws but does not by itself settle the metaphysical question of what grounds a system's capacity to change.

Even a fully successful First Way establishes only a first, unmoved actualizer of change; it says nothing directly about personhood, moral perfection, or uniqueness, all of which Aquinas argues for separately using divine simplicity and other considerations.

It also does not address whether the universe began to exist in time, a separate question addressed by different arguments (such as the Kalam) and by empirical cosmology.

When presenting this argument, immediately clarify the technical sense of 'motion' as actualization of potency, since most objections in casual conversation arise from assuming a Newtonian sense the argument does not intend.

Acknowledge candidly that the argument's persuasiveness depends on accepting Aristotelian metaphysics as a live, defensible framework, which is a substantive philosophical commitment that many contemporary physicists and philosophers do not share, and be ready to discuss why Thomists think that framework remains defensible independent of any particular physical theory.