The unmoved first mover

Aristotle 384 BC · 322 BCObserve, classify, reason: happiness through virtue.

What it claims

There exists a first mover that moves without being moved, eternal and fully actual.

Why

Whatever moves is moved by something else, and the chain cannot go back forever: there must be a first mover that is not moved.

In short

"The unmoved first mover" is an argument by Aristotle (Metaphysics, Λ 6-7) on the question "Does God exist?" (answer: theism). Its claim: There exists a first mover that moves without being moved, eternal and fully actual. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P3). AskPhilosophy presents 2 objections, 2 of them with a documented reply, and the original text behind each premise, checked word for word.

Metaphysics, Λ 6-7

Valid with an implicit premise

The motion of the heavens never stops. What sustains it? Aristotle concludes that there is an eternal reality that moves without being moved.

click a premise to reject it

If you accept every premise, logic forces you to accept the conclusion. So the debate is about the premises.

Checked by a logic solver: the premises cannot all be true while the conclusion is false.
Without the implicit premise: Without P3, the computer finds a case where each mover is moved by another, to infinity, with no first mover.

With P3 the conclusion follows (P4 is only the rule linking everything). Without it, an infinite chain of moved movers would be enough to explain eternal motion. Aristotle defends this point in the Physics, but it is the most contested premise.

Analogy. A row of dominoes keeps falling forever. If each is pushed by the one before, what pushes the whole? Aristotle answers: something outside the row, which attracts without being pushed, as we are drawn by what we love.

Objections

Each objection is a conversation: click to see who replies and where the debate stands.

Hume 1779 · attacks premise 3

If each element of an infinite chain is explained by the previous one, asking for a cause of the whole chain is an arbitrary addition by the mind.

a defender replies

summary: their text is not yet in our corpus

No reply possible: objection raised in 1779, after the author's death (-322).

Thomas Aquinas defender of the argument · 1268

Summary: in the first way (Summa theologiae, I, q. 2, a. 3), Aquinas takes up Aristotle's argument for a series of movers in which each moves only because it is moved by the first, as the stick moves only because the hand moves it. Without a first mover, no intermediate mover moves, however long the chain. (Aquinas grants, however, that a series of causes in time, like fathers and sons, could be infinite: Summa theologiae, I, q. 46, a. 2.)

Where the debate stands (our summary)Hume asks why an infinite chain would need a cause beyond its members. Aquinas answers that in a chain where each member acts only in virtue of the previous one, length changes nothing. The core disagreement is whether this kind of dependence really describes the world (see also the debate on whole and parts in Leibniz and Russell).

Modern physics (Newton) 1687 · attacks premise 2

By the principle of inertia, a body in motion keeps moving without a mover: motion needs no sustaining cause, only its change does.

a defender replies

summary: their text is not yet in our corpus

No reply possible: objection raised in 1687, after the author's death (-322).

Edward Feser defender of the argument · 2012

Summary: Feser, a contemporary defender of the Aristotelian proof, argues that the two principles do not contradict each other. The principle of inertia says that a body continues in uniform motion, but not why. And "motion" in Aristotle and Aquinas means any passage from potentiality to actuality, not only displacement: even if uniform motion needed no mover, the argument could start from other changes (acceleration, for example).

Where the debate stands (our summary)The principle of inertia removes the need for a mover to sustain motion in space; Aristotle's defenders answer that the argument concerns any passage from potentiality to actuality. The core disagreement is whether this Aristotelian notion of change still has a useful sense in modern physics.

What today's voices say

Defend it

Bishop Robert BarronIn a Word on Fire commentary he treats Aquinas's argument from motion as hard to refute. 'Motion' means any change, and the unmoved mover is pure act, which 'simply is'.