Lecture 9: Why Plato's soul arguments fail, and the turn to physicalism

What this lecture covers

This lecture wraps up the course's critical examination of Plato's Phaedo and marks a turning point: from here on, the course proceeds on physicalist assumptions. Kagan first tidies up loose ends on the argument from simplicity, questions whether the soul is really changeless or simple at all, and revisits Plato's objections to the harmony analogy for the mind. He then reconstructs a final argument, the argument from essential properties, and shows where it trades on an ambiguity in the word "deathless."

By the end, you should be able to explain why none of Plato's arguments for an immortal soul succeed, and why Kagan thinks he is nonetheless justified in concluding physicalism is true without personally disproving the existence of souls.

Key ideas

• Soul may not be changeless: beliefs, desires, and goals all seem to change over time, which undercuts the premise that the soul is changeless and therefore simple.
• Plato's own view complicates simplicity: in The Republic, Plato argues the soul has three parts (rational, spirited, appetitive), which conflicts with treating it as simple elsewhere.
• Simples could still vanish: even a partless thing could conceivably pop out of existence, so simplicity alone does not guarantee immortality.
• Harmony as an early physicalist analogy: the suggestion that mind relates to body as harmony relates to a harp anticipates the idea that mental activity is produced by a properly functioning body.
• Plato's four objections to the harmony analogy: harmony can't precede the instrument, harmony comes in degrees, harmony can't itself be harmonious, and harmony can't causally affect the instrument; Kagan argues each objection ultimately fails or misses the point of an analogy.
• Essential vs. contingent properties: an essential property is one an object must have to exist at all (fire must be hot); a contingent property is one it happens to have but could lack (a car happens to be blue).
• The argument from essential properties: if being alive (capable of thought) is essential to the soul, the soul is 'deathless'; Plato infers indestructibility from this.
• Ambiguity in 'deathless': it could mean the soul is never simultaneously existing and dead (which the essential-property claim supports), or that the soul can never be destroyed (which it does not support), and Plato's argument conflates the two.

Walkthrough

Loose ends on the argument from simplicity (0:00)

Kagan revisits the argument from simplicity from the previous lecture, noting two further problems: the soul's beliefs and desires seem to change constantly, which undermines the claim that it is changeless, and Plato himself, in The Republic, argues the soul has multiple parts, undercutting the simplicity claim. He also points out that even a truly simple thing could still, in principle, pop out of existence, so simplicity would not guarantee immortality even if granted.

Plato's objections to the harmony analogy (5:56)

Returning to the suggestion that the mind is like the harmony of a well-tuned body, Kagan works through four objections Plato raises against this physicalist-flavored analogy: harmony cannot exist before the harp does (while the soul, per the recollection argument, predates the body), harmony admits of degrees while souls seemingly do not, harmony cannot itself be harmonious, and harmony cannot causally affect the instrument the way the soul affects the body. Kagan responds to each, arguing that mental traits do come in degrees, that harmonies can vary in kind, and that on a physicalist view "the soul affecting the body" is really one bodily part (the brain) affecting another (the muscles), which has a rough parallel in how one vibrating string can set others vibrating.

The argument from essential properties (19:42)

Kagan introduces the distinction between essential properties (which an object must have to exist, like fire being hot) and contingent properties (which it happens to have, like a car being blue). He grants Plato's claim that being alive, or capable of thought, is an essential property of the soul, and reconstructs the resulting argument: since the soul is essentially alive, it is "deathless," and since it's deathless it can't die, so it can't be destroyed. Kagan then identifies the flaw: "deathless" can mean either that the soul is never simultaneously alive and dead, or that the soul can never be destroyed at all, and only the weaker first sense follows from the essential-property premise.

There is no good reason to believe in souls (37:06)

Having found every argument for an immaterial or immortal soul unpersuasive, Kagan announces he will proceed on the assumption that physicalism is true for the rest of the course. He anticipates the objection that this places an unfair burden of proof only on the dualist, and argues that disbelief does not require disproof, using dragons, unicorns, and Zeus as examples: you are justified in disbelieving something once you've undermined the arguments offered for it, without needing to prove its impossibility.

Qualifications and conclusion (47:53)

Kagan closes by clarifying that he does not think souls are impossible or incoherent, only that the case for them is weak. He compares this to not believing in unicorns despite them being logically possible. From this point forward, the course will examine death strictly from the physicalist perspective, treating talk of the mind as talk about what a properly functioning body can do.

Before you watch

• Watch the previous lecture on the argument from simplicity, since this lecture directly continues and revises that discussion.
• Be familiar with the basic contrast between dualism and physicalism, and the argument from recollection, both covered earlier in the course.

Check your understanding

1. Why does the fact that beliefs and desires change pose a problem for the claim that the soul is simple?
2. Summarize Plato's four objections to treating the mind as the harmony of the body, and Kagan's response to each.
3. Give an example each of an essential property and a contingent property, in your own words.
4. Explain the two different senses of 'deathless' Kagan distinguishes, and which one Plato's argument actually establishes.
5. Why does Kagan think he does not need to disprove the existence of souls in order to be justified in disbelieving in them?