4.3 Introduction to Cartesian Dualism

What this lecture covers

This lecture turns to Cartesian dualism: the view that mind and body are two genuinely distinct substances, matter whose essence is extension and mind whose essence is thinking. Millican works through Descartes's arguments for this claim from the Discourse on Method and the Sixth Meditation, identifies a fallacy in the simpler version, and evaluates the more sophisticated version alongside two classic objections to dualism.

You'll come away able to state what Cartesian dualism claims (distinct substances, not just distinct properties), explain the Hesperus/Phosphorus-style fallacy in Descartes's simple argument, and describe the interaction problem and the causal closure objection.

Key ideas

• Cartesian dualism defined: the claim that mind and body are distinct substances, not merely that they have different properties.
• The discourse argument and its fallacy: "I can doubt my body exists, I cannot doubt I exist, therefore I am not my body" resembles the fallacious inference that Hesperus and Phosphorus must be different because one can be doubted to be the other, since "being doubted by me" is a fact about the doubter, not a genuine property of the object.
• The Sixth Meditation argument: Descartes claims he can clearly and distinctly conceive of himself as a non-extended thinking thing and of body as an extended non-thinking thing, and infers that God could therefore create them as genuinely separate substances.
• The epistemology-to-metaphysics move: Millican argues Descartes illegitimately slides from "I only know for certain that I think" to "my essence is to think," a move he calls generally suspicious.
• The thinking stone thought experiment: a stone that God made capable of thought could reason the same way Descartes does and wrongly conclude its essence is to think.
• Possibility and knowledge: Millican distinguishes epistemic possibility ("for all I know, X could happen") from real metaphysical possibility, arguing Descartes's argument only supports the weaker, epistemic claim.
• The interaction problem: if mind is purely non-extended and body purely extended, it is unclear how the two substances could ever causally interact.
• The causal closure objection: since only physical events seem to cause physical events, dualism appears to require denying this, though Millican argues the principle itself is more an assumption of science than an established fact.

Walkthrough

Defining Cartesian dualism (0:10)

Millican states the core claim precisely: to be a Cartesian dualist you must hold that mind and body are distinct substances, not merely that they have different properties. He notes this is a stronger and more substantial claim than it might first appear.

The discourse argument and its fallacy (1:11)

Millican presents Descartes's simpler argument from the Discourse on Method: since he can doubt his body's existence but cannot doubt his own existence, mind and body must be distinct. Using the Hesperus/Phosphorus example (two names for Venus that the ancients didn't know were the same object), he shows this argument is fallacious, because "being doubted by me to be X" is not a genuine property of the object in question but a fact about the doubter's state of knowledge.

The Sixth Meditation argument (5:24)

A more sophisticated version appears in the Sixth Meditation: Descartes claims a clear and distinct understanding of himself as a non-extended thinking thing and of body as extended and non-thinking, then argues God's power to create these as separate confirms they are genuinely distinct. Millican questions the move from epistemic certainty about thinking to a metaphysical claim about essence, using the example of a stone God made capable of thought, which would wrongly conclude by the same reasoning that thinking is its essence.

Possibility, knowledge and the final move (11:32)

Millican examines Descartes's final step—that since God could create mind and body separately, they must actually be distinct—more sympathetically, but shows the argument only works if "possibility" is read in a strong, real sense rather than a merely epistemic one ("for all I know"). Returning to the Hesperus/Phosphorus example, he argues that what seems possible given our ignorance is not the same as what is genuinely possible given the underlying facts, so Descartes's argument for dualism ultimately falls short.

The interaction problem (15:38)

Millican introduces the classic objection that it is unclear how a non-extended thinking substance and an extended non-thinking substance could interact at all. He notes this is a particular problem for Descartes because Descartes thought causation itself must be intelligible, but observes that on a Humean view of causation—where causal connections are just observed regularities rather than intelligible necessities—the interaction problem loses some of its force, even though formulating actual laws linking mental and physical events remains a real challenge.

The causal closure problem (18:40)

The lecture closes with the causal closure objection: if only physical events cause physical events, dualism seems to require denying this, since mental choices like deciding to raise an arm appear to cause physical effects. Millican argues this principle is more an assumption or ambition of science than an established result, comparing confident claims of physical determinism to Einstein's unsupported conviction that "God doesn't play dice." He suggests a stronger worry for dualism is instead the difficulty of explaining how an immaterial mind could have evolved.

Before you watch

• Watch the two lectures on scepticism of the external world, since Millican builds on Descartes's method of doubt established there.
• Recall Hume's view of causation from the earlier lectures on induction, which Millican draws on when discussing the interaction problem.

Check your understanding

1. What must you accept, beyond mind and body simply having different properties, to count as a Cartesian dualist?
2. Why is Descartes's argument "I can doubt my body exists, I cannot doubt I exist, therefore I am not my body" fallacious?
3. How does the thinking-stone example challenge Descartes's move from "I know I think" to "my essence is to think"?
4. What is the difference between epistemic possibility and real possibility, and why does it matter for Descartes's Sixth Meditation argument?
5. Why does Millican think the causal closure principle is less damaging to dualism than the objection from evolution?