Lecture 6: The Cartesian Argument Refuted; Plato's Forms, Part I

What this lecture covers

This lecture finishes evaluating Descartes's argument that imagining the mind without the body proves they are distinct things. Kagan gives a counterexample using the Morning Star and Evening Star (both actually Venus) to show that being able to imagine A without B doesn't prove A and B are different things, and discusses several competing diagnoses of exactly where such arguments go wrong.

The lecture then turns to Plato's Phaedo, previewing why Socrates faces his execution cheerfully, and introduces the core of Plato's metaphysics: the theory of Forms, which will underpin the arguments for the soul's immortality covered in later lectures.

Key ideas

• The identity test: if you can imagine A existing without B, that's normally taken to show A and B are two different things, not one.
• The Morning Star/Evening Star counterexample: you can imagine a world with the Evening Star but no Morning Star, yet both names refer to the same object, Venus, showing the imagination test can mislead.
• Competing diagnoses: philosophers disagree about why the Cartesian-style argument fails, whether we don't really imagine what we think we imagine, whether imagination is an unreliable guide to possibility, or whether identity itself can be contingent.
• The Phaedo's real question: the dialogue mostly assumes the soul exists and instead argues about whether it survives the body's death and whether it is immortal.
• Socrates' division of labor: unlike modern dualists, Socrates assigns sensations, desires and emotions to the body, reserving pure rational thought for the soul.
• Platonic Forms: perfect, abstract concepts like Beauty itself, Justice itself or Circularity itself, which ordinary objects only imperfectly 'participate in,' and which only the mind, not the senses, can grasp.
• The mathematics analogy: numbers are not physical objects you can see or touch, yet the mind can think about them, offering a model for how the mind grasps other Forms.
• Plato's heaven: Socrates' belief that after death his soul will have direct contact with the Forms, explaining his calm, even happy, mood before his execution.

Walkthrough

Refuting the Cartesian argument (0:01)

Kagan reviews the Cartesian thought experiment and introduces the Evening Star / Morning Star counterexample: since both refer to Venus, you cannot actually imagine a coherent world with one but not the other, even though it seems easy at first, which parallels the flaw he thinks the mind-body argument has.

Where exactly the argument breaks down (11:16)

He walks through several candidate explanations for why the astronomy case fails, misdescribing what you imagined, imagination being an unreliable guide to logical possibility, or identity being contingent, and notes philosophers disagree on which applies, making Descartes's argument at least unreliable if not simply wrong.

Setting up the Phaedo (18:26)

Kagan reframes the upcoming reading: the Phaedo mostly takes the soul's existence for granted and instead debates whether it survives bodily death and whether it is immortal, since even interactionist dualism leaves open that the soul could die along with the body.

Socrates' picture of body and soul (23:36)

Unlike the modern dualists discussed earlier in the course, Socrates assigns sensations, cravings and emotions entirely to the body, reserving pure rational thought, especially thought about abstract ideas, for the soul.

Introducing Platonic Forms (25:41)

Using examples like beauty, justice and circularity, Kagan explains Plato's view that perfect, abstract Forms exist beyond the physical world, that ordinary things only partially participate in them, and that only the mind, not the senses, can grasp them. He extends the point using mathematics: numbers aren't physical objects, yet the mind clearly thinks about them.

Plato's heaven (30:49)

The lecture closes by explaining why Socrates, on the day of his execution, is cheerful rather than distressed: he believes his soul, freed from bodily distraction, will ascend to a realm where it has direct contact with the Forms, setting up the arguments for immortality the course will examine next.

Before you watch

• This lecture directly concludes the Cartesian argument introduced in Lecture 5; watch that lecture first.
• Begin or continue reading Plato's Phaedo, as this lecture introduces the metaphysical background needed to follow it.

Check your understanding

1. Why does the fact that the Evening Star and Morning Star are both Venus undermine the general form of Descartes's argument?
2. What are the different proposed explanations for why the Cartesian-style argument fails, and why is it hard to settle which one is correct?
3. How does Socrates' division between what the body handles and what the soul handles differ from the modern dualist arguments covered earlier in the course?
4. According to Plato's theory of Forms, why can't the number two, or perfect justice, be found anywhere in the physical world, and how does the mind supposedly still grasp them?