QuestionsIs the mind the brain?The soul is made of atoms
Is the mind the brain?The mind comes from matter

The soul is made of atoms

The atomic soul

Democritus c. 460 BC · c. 360 BCNothing exists but atoms and the void.
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What it claims

Soul and thought are matter in motion: the mind comes from matter.

Why

Everything is atoms and void; the soul is an assembly of atoms, the same as intelligence: thought is matter in motion.

In short

"The soul is made of atoms" is an argument by Democritus (Diogenes Laertius IX, 44 · Aristotle, On the Soul I 2 (405a)) on the question "Is the mind the brain?" (answer: the mind comes from matter). Its claim: Soul and thought are matter in motion: the mind comes from matter. According to AskPhilosophy's logic check (Z3 solver), the argument is valid only if one grants a premise the author leaves unstated (P4). AskPhilosophy presents 3 objections, 0 of them with a documented reply, and the original text behind each premise, checked word for word.

Diogenes Laertius IX, 44 · Aristotle, On the Soul I 2 (405a)
Valid with an implicit premise

If everything is atoms and void, the soul must be too. That is what Democritus concludes, according to Diogenes Laertius and above all Aristotle, who sets out his theory in order to discuss it. For both, the soul is made of the finest atoms, smooth and round, like those of fire, and it is the same thing as intelligence. We have no text of Democritus on this point: the argument is a reconstruction from these two witnesses.

P1The first principles of the universe are atoms and the void; everything else is merely thought to exist. (Diogenes Laertius, IX, 44)
P2The soul belongs to the primary, indivisible bodies: it moves the body because of the fineness and shape of its atoms, spherical and very mobile, like those of fire. (Aristotle, On the Soul I 2)
P3The soul is not distinct from intelligence: they are the same thing. (Diogenes Laertius, IX, 44)
P4 · implicit premiseIf everything is atoms and void, and if the soul is an assembly of atoms identical with intelligence, then thought is nothing more than matter in motion: there is no separate substance.
Therefore soul and thought are matter in motion: the mind comes from matter.

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 P4, the computer finds a case where everything is atoms, the soul is made of atoms and is intelligence, but where thought is not reducible to matter in motion (an assembly of atoms would have properties no atom has): the conclusion falls.
The argument is valid with P4. P1, P2 and P3 give three facts: everything is atoms, the soul is made of atoms, the soul is intelligence. P4 draws from them that thought is only matter: this is a premise we add out of charity, because no witness writes it in this form. It is plausible, but can be contested: one would have to show that an assembly of atoms can really think, and that is precisely what Democritus is reproached with.
Analogy. A screen displays pictures, yet it is made of tiny points of light. Democritus would say thought is to the soul what the picture is to the screen: an effect of small elements put together, not something of another nature.

Objections

3

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

Aristotle (On the Soul I 3) 335 BC · attacks premise 2

If the soul is made of atoms that never stop moving and drag the body along with them, how can it also produce rest? Aristotle judges it difficult, even impossible, to say. And in general, this is not how the soul appears to move the animal: it does so through intention and thinking.

no reply
No reply possible: objection raised in -335, after the author's death (-360).
The debate goes on (our summary) Democritus died before this passage and we have no answer from him. A defender of atomism could say that an intention is itself a motion of atoms, but no witness in our corpus says so: this is our reconstruction, not a text.
Cicero (Tusculan Disputations I) 45 BC · attacks the conclusion

Cicero sets Democritus aside, while calling him "a very great man": he sums up his thesis (the soul is formed by a fortuitous concourse of small, light and round bodies) and adds, ironically, that for these people there is nothing a crowd of atoms cannot do. He develops no argument: he judges the thesis not worth discussing. The implied objection is that a chance assembly is not enough to make a thinking soul.

no reply
No reply possible: objection raised in -45, after the author's death (-360).
The debate goes on (our summary) Cicero writes "fortuitous concourse", but Diogenes Laertius reports that for Democritus "all things happen by necessity": the vortex of atoms causes everything (IX, 45). If this second witness is right, the chance objection does not hit the real thesis. But he too writes six centuries after Democritus, without quoting him.
Voltaire (Philosophical Dictionary, "Soul") 1764 · attacks the conclusion

Voltaire, quoting Epicurus' formula "atoms think in us", asks the awkward question: how does an atom think? He asks us to admit we know nothing about it. The objection does not say the soul is immaterial: it says the atomic explanation does not show the step from matter to thought.

no reply
No reply possible: objection raised in 1764, after the author's death (-360).
The debate goes on (our summary) No text of Democritus says how an assembly of atoms senses or thinks: the witnesses speak of the shape and motion of the soul's atoms, not of this step. Voltaire targets atomism as a whole, and his answer "admit you know nothing" echoes Democritus' own "we know nothing" about what things are (B 117).