Priority of Soul (the Laws X Argument for the Gods)

In Book X of the Laws, Plato gives the corpus's fullest natural theology: the proof that the gods exist, grounded in the priority of soul over body. Soul is defined as self-moving motion — "the motion capable of moving itself" (896a), the only motion that can be first — hence the archē of all change and "older than matter." The ordered revolution of the heavens, "regular, uniform … like a sphere being turned on a lathe" (898a–b), shows the cosmos is governed by the best soul, reason — so "everything is full of gods." This refutes the materialist atheist (who makes soul, art, and law late and conventional) and grounds the law against impiety. The Book-X soul is cosmic — the self-moving first cause — and must not be conflated with the psychological tripartite soul.

Key Points

The materialist target (885b–890a): the impious hold that fire, water, earth, air are primary "by nature and chance," the heavenly bodies "just earth and stones," and gods and justice therefore artificial — "there is no natural standard of justice at all" (889e), so "the true natural life is a life of conquest." This is convention-vs-nature conventionalism at its metaphysical root.
The classification of motions (894b–895b): a motion moved by another can never be first (no antecedent could start the chain); only a motion "capable of moving both itself and other things" can be the original source.
Soul = self-mover (896a): that self-moving source is what we call soul; therefore "soul is prior to matter, and matter takes second place" (896c), and reason, art, law, character are "preeminently natural."
From order to reason (896d–899d): the heavens' regular motion has "the closest possible affinity" to the cyclical motion of reason; disorder would betoken an evil soul, but the heavens are orderly — so they are governed by the best soul.
Providence and incorruptibility (899d–907b): the gods neither neglect human affairs (the craftsman tends even the smallest parts; "you exist for the sake of the universe") nor can be bribed (a bribable guardian is a watchdog who takes a cut from the wolves).

What the Concept Does

Inverts the materialist ontology — by making soul the first cause, it makes mind, law, and value prior to and more natural than the physical elements, dissolving the nomos/phusis split (rightly grounded nomos just is phusis).
Grounds theology in argument, not tradition — Socrates/the Athenian "has no truck with the authority of myths or poets"; the gods are reached by a proof about motion.
Licenses the impiety law — but graded: the merely foolish atheist gets reformatory imprisonment (the sōphronistērion), not death.

Stakes

This is the metaphysical foundation under the Laws' politics: "God, not man, is the measure of all things" (716c) and the rule of law-as-reason both presuppose that reason/soul is the natural sovereign. It is also the corpus's strongest answer to the conventionalist — where the Gorgias and Republic answer Callicles and Glaucon on ethical-psychological grounds, the Laws attacks the cosmology underneath them.

Connections

contrasts with tripartite-soul — the Book-X soul is the cosmic self-moving first cause, NOT the psychological trichotomy (rational/spirited/appetitive); the shared word psuchē hides a categorial difference.
contrasts with the Alcibiades I's minimal soul-model — the cosmic self-mover and the (disputed) Alcibiades' unitary self-as-ruler are two more psuchē concepts beside the tripartite-soul: three soul-models answering three different questions. See claims#plato-self-is-soul-minimal-model (candidate).
requires nomos-phusis — the proof's purpose is to refute the conventionalist by showing soul prior to body, making law and justice "preeminently natural."
shares mechanism with plato-timaeus — the ordered cosmos governed by soul/reason (the world-soul, the demiurge's ordering) reappears here in argumentative rather than mythic form.
is re-deployed by the Epinomis — the (disputed) appendix to the Laws carries the priority-of-soul premise forward but bends it to prove the stars are intelligent gods (982): same premise, new astral-theological conclusion (a dubium).
requires the-mean — "God, not man, is the measure" (716c) makes the divine/measured the standard, sharpening measure's anti-Protagorean edge.

Open Questions

The inference from "self-moving" to "soul" to "divine reason" is the load-bearing and most contestable step: one might grant a first mover yet deny it must be psychic or good. (The text even allows "more than one soul," 896e — room for an evil world-soul.)
Cross-tradition: how far is this a forerunner of Aristotle's unmoved mover and the later cosmological/design argument? The note flags Anaxagoras as the half-right precursor (reason orders the heavens, but he "went astray over the soul's priority to matter," 967b). Flagged, not asserted.

Sources

plato-laws — the materialist target (885b–890a); soul as self-mover (894b–896c); order → reason (896d–899d); providence and incorruptibility (899d–907b).
plato-timaeus — the world-soul and the demiurge's ordering of the cosmos (the mythic counterpart).
plato-alcibiades-1 — contrast: the unitary self-as-soul (user/ruler of the body, 129b–130c) — a third soul-model beside the cosmic self-mover and the tripartite soul (authorship disputed).
plato-epinomis — the Laws X priority-of-soul re-deployed to prove the stars are intelligent gods (980e–982). A dubium.