QuestionsIs the mind the brain?What thinks is a body
Is the mind the brain?The mind comes from matter

What thinks is a body

Thomas Hobbes 1588 · 1679Without a state, life would be solitary, brutish and short.
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What it claims

The thing that thinks is a body: the mind is nothing but motion in certain parts of the body.

Why

Thinking is an act, which needs a subject; that subject is a substance, and substance and body are the same thing.

In short

"What thinks is a body" is an argument by Thomas Hobbes (Third Objections (1641) and Leviathan, ch. 1 and 34) on the question "Is the mind the brain?" (answer: the mind comes from matter). Its claim: The thing that thinks is a body: the mind is nothing but motion in certain parts of the body. According to AskPhilosophy's logic check (Z3 solver), the conclusion does follow from its 3 premises. AskPhilosophy presents 3 objections, 2 of them with a documented reply, and the original text behind each premise, checked word for word.

Third Objections (1641) and Leviathan, ch. 1 and 34
Valid

In 1641 Descartes circulated his Meditations: I think, therefore I am a thinking thing, a mind distinct from the body. Hobbes, then in Paris, accepted "I think, therefore I am" but refused the rest. A thought needs a subject that thinks, and nothing says that subject is not a body. Ten years later, in Leviathan, he went further: an "incorporeal substance" is a contradiction in terms.

P1Thinking is an act, and no act can be conceived without a subject doing it: no thought without a thing that thinks, as no walk without someone walking.
P2The subject of an act, what stays the same under its changes, is a substance.
P3"Substance" and "body" mean the same thing: an incorporeal substance is a contradiction, like a round square. Everything in the universe is body.
Therefore the thing that thinks is a body: the mind is nothing but motion in certain parts of the body.

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.
The argument is valid with no hidden premise, and all three premises are essential. Descartes accepts P1 and P2: he says himself that every thought presupposes a substance that thinks. So the whole disagreement is about P3. For Hobbes, "substance" means "body"; for Descartes there are two kinds of substance, one extended, the other thinking. The question becomes: is P3 a discovery about the world, or a mere definition that settles the debate in advance?
Analogy. Hobbes compares the heart to a spring, the nerves to strings and the joints to wheels. A watch "tells" the time without a soul inside the watch: it is parts in motion. For him the same goes for our sensations and thoughts, just with much finer parts.

Objections

3

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

Descartes 1641 · attacks premise 3

The subject of thought is indeed a substance, a "metaphysical matter" if you like, but not therefore a body. Almost everyone distinguishes spiritual from corporeal substances. And acts of thought (understanding, willing, imagining, sensing) have no affinity with acts of the body (size, shape, motion): thought differs wholly from extension.

Hobbes replies
Hobbes Hobbes's reply in Leviathan (1651), without naming Descartes
An "incorporeal substance" is like an "incorporeal body": two words whose meanings contradict each other and which together mean nothing, like a round quadrangle.
The debate goes on (our summary) Descartes would answer that Hobbes proves nothing: he decides that "substance" means "body", then observes that a non-bodily substance is contradictory. The Cartesian starts from what he knows: he clearly conceives thought without extension (see Descartes' argument on the distinction of mind and body).
Leibniz 1714 · attacks premise 3

Imagine a thinking machine, enlarged so that one could walk into it as into a mill. Inside we would find only parts pushing one another, and never anything to explain a perception. Bodies in motion do not explain feeling something.

no reply
No reply possible: objection raised in 1714, after the author's death (1679).
The debate goes on (our summary) A materialist would answer that the argument only proves our ignorance: walking through a mill you do not see the flour "being" flour either. But Leibniz's question has stayed alive: today it is called the "hard problem of consciousness".
Objection anticipated by Hobbes attacks the conclusion

If everything is body, then God and the angels, whom religion calls spirits, are nothing at all.

Hobbes replies
summary: their text is not yet in our corpus
Hobbes
It does not follow that spirits are nothing: they have dimensions, and so are really bodies, merely invisible ones. The name "incorporeal" is a title of honour given to God out of piety, not a description of his nature, which is incomprehensible.
The debate goes on (our summary) His opponents at the time saw this as disguised atheism: a bodily God is no longer the theologians' God. Hobbes maintained it was a faithful reading of Scripture.