QuestionsIs the mind the brain?One thing, two attributes: the mind is the idea of the body
Is the mind the brain?The question is badly put

One thing, two attributes: the mind is the idea of the body

Spinoza's parallelism

Benedictus de Spinoza 1632 · 1677God, that is, Nature: everything follows by necessity.
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What it claims

Mind and body are one and the same thing, conceived now under the attribute of thought, now under that of extension. Neither acts on the other: the order of the body's actions and that of the mind's actions are the same order, seen in two ways.

Why

The mind is the idea of the body, and a thing and its idea are one reality seen in two ways.

The same idea through history
Benedictus de Spinoza1677Mind and body are one and the same thing conceived under two attributes, thought and extension.
Antonio Damasio2003The neurologist Antonio Damasio rereads Spinoza to think of emotions as states of body and brain (Looking for Spinoza, 2003).
In short

"One thing, two attributes: the mind is the idea of the body" is an argument by Benedictus de Spinoza (Ethics II, prop. 7 (scholium) and 13; III, prop. 2 (scholium), 1677) on the question "Is the mind the brain?" (answer: the question is badly put). Its claim: Mind and body are one and the same thing, conceived now under the attribute of thought, now under that of extension. Neither acts on the other: the order of the body's actions and that of the mind's actions are the same order, seen in two ways. According to AskPhilosophy's logic check (Z3 solver), the conclusion does follow from its 2 premises. AskPhilosophy presents 3 objections, 2 of them with a documented reply, and the original text behind each premise, checked word for word.

Ethics II, prop. 7 (scholium) and 13; III, prop. 2 (scholium)
Valid

Descartes said: mind and body are two things, which act on each other. Others say: the mind is just the brain. Spinoza rejects both. For him there is only one reality, which can be described in two ways: as thought or as extension (matter in motion). The human mind is the idea of the body: it is the same thing, seen from two sides. So the question "is the mind the brain?" is badly posed, because it acts as if we had to choose between two things, or reduce one to the other. There are not two things, and not one thing swallowing the other.

P1The object of the idea constituting the human mind is the body, that is a certain mode of extension actually existing, and nothing else. In other words, the human mind is the idea of the human body.
P2A mode of extension and the idea of that mode are one and the same thing, expressed in two ways. Spinoza's example: a circle in nature and the idea of that circle are one thing, explained through different attributes, and we find the same order of causes under either.
So mind and body are one and the same thing, conceived now under the attribute of thought, now under that of extension. Neither acts on the other: the order of the body's actions and that of the mind's actions are the same order, seen in two ways.

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.
Valid. The two premises suffice. The difficulty lies elsewhere: by "one and the same thing", does Spinoza mean a strict identity, or only two parallel descriptions that correspond? Commentators disagree. The scholium of II, 7 says "one and the same thing"; our reconstruction follows this most literal reading. P2 is also a point of attack: one may refuse that an idea is the same thing as its object.
Analogy. Think of a coin: heads and tails are not two objects, and nobody asks whether "heads" is the same thing as "tails". It is the same coin, seen from two sides. For Spinoza, thought and extension are like two sides of one reality (the comparison is ours, not Spinoza's, who speaks of "two attributes" and not two faces).

Objections

3

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

Descartes (Letter to Elisabeth, 21 May 1643) 1643 · attacks the conclusion

For Descartes, soul and body are two distinct things. We have three primitive notions: that of the body (extension), that of the soul (thought) and that of their union, on which depends the power of the soul to move the body and of the body to act on the soul. In his system this union is a fact we know, but cannot explain by the other two notions.

Spinoza replies
E
Elisabeth of Bohemia
Elisabeth had already asked Descartes how the human soul, being only a thinking substance, can determine the spirits of the body to perform voluntary actions.
Spinoza Spinoza's reply on the pineal gland
Spinoza mocks the idea that the soul pushes the pineal gland: he sees in it a hypothesis "beside which occult qualities are commonplace", whereas Descartes blamed the scholastics for explaining the obscure by the occult. He also asks what clear and distinct conception Descartes has of the union of a thought with a small piece of extension, and concludes that, having conceived the soul so distinct from the body, Descartes could explain this union only through God.
See the rest of the debate (2)
Spinoza The same question, put plainly
Spinoza asks precisely what Descartes understands by the union of mind and body.
Spinoza What the body can do
Spinoza adds that the idea that the mind commands the body comes from our ignorance of what the body can do by the laws of nature alone: no one has yet established what the body can or cannot do without being determined by the mind.
The debate goes on (our summary) Descartes himself admitted (letter of 28 June 1643) that the union of soul and body is better known through the senses and ordinary life than through metaphysics, and that the human mind has difficulty conceiving both their distinction and their union. Modern interactionist dualists answer Spinoza that the mystery of interaction is no proof that it does not exist, and that two-attribute monism has its own mystery: why does the same thing have two ways of being described?
Leibniz (Monadology) 1714 · attacks the conclusion

Leibniz explains the agreement of soul and body differently from Spinoza: the soul follows its own laws, the body its own, and they agree "in virtue of the pre-established harmony" between all substances. Both are real, distinct substances, which do not act on each other, but God has set them in advance to agree.

no reply
No reply possible: objection raised in 1714, after the author's death (1677).
The debate goes on (our summary) Spinoza died in 1677, long before the Monadology, and could not reply. A Spinozist would say that Leibniz needlessly doubles what has to be explained: if mind and body are one thing, their agreement does not need to be arranged beforehand by God. A Leibnizian answers that, in Spinoza, each being is only a mode of a single substance, which removes the real individual. Both systems want to explain the agreement without interaction, but they disagree on what exists.
Ehrenfried von Tschirnhaus (Letter 65 to Spinoza) 1675 · attacks premise 2

If body and mind are one modification expressed in infinite ways (through thought, through extension, through other attributes unknown to us), why does the mind perceive only the expression of that modification in extension, that is the human body, and no other?

Spinoza replies
Spinoza Spinoza's reply (letter 66)
Spinoza answers that the infinite ideas that express a thing in God's understanding do not form one mind, but infinitely many minds, since these ideas have no connection with one another.
The debate goes on (our summary) This reply is very short: Spinoza refers his correspondent to two passages of the Ethics. Many commentators find that it leaves open in what sense these other "minds" are ours, and why we are one particular mind.