Lecture 8: Plato's argument for the soul's simplicity

What this lecture covers

This lecture continues the course's look at Plato's Phaedo and its case for the immortality of the soul. Kagan first closes out two earlier arguments (recollection and recycling), showing that even combined they only prove the soul existed before birth, not that it survives death. He then reconstructs what he calls the argument from simplicity: things with parts can be destroyed, only changing things have parts, and invisible things do not change, so invisible things cannot be destroyed. Since the soul is invisible, the argument concludes it cannot be destroyed.

By the end, you should be able to state the argument from simplicity in its premise-and-conclusion form, and explain why Kagan thinks it ultimately fails once "invisible" is pinned down carefully.

Key ideas

• Argument from recycling: things get built from pre-existing parts, and when destroyed, decompose back into those parts, so the parts persist through the object's destruction.
• Argument from recollection: our grasp of perfect forms (justice, roundness) implies the soul knew the forms before birth, since nothing in the physical world is perfectly just or round.
• Combining the two arguments: only shows the soul existed before birth, not that it survives death, because a pre-existing part (like a car's engine) can still eventually be destroyed.
• Argument from simplicity: only composite (multi-part) things can be destroyed; only changing things are composite; invisible things don't change; so invisible things can't be destroyed; the soul is invisible; therefore the soul can't be destroyed.
• Cebes' objection: Socrates only earns the weaker conclusion "indestructible, or nearly so," which is not enough for immortality, since even something very durable eventually wears out.
• Simmias' harmony objection: harmony is invisible (you can't see it) yet destructible (smash the harp), so invisibility does not guarantee indestructibility.
• Three senses of 'invisible': cannot be seen, cannot be sensed by any of the five senses, or cannot be detected at all (even indirectly, through effects).
• The radio wave counterexample: radio waves cannot be sensed by any of the five senses, yet they are destructible and detectable through their effects, which undermines the argument once "invisible" is defined as undetectable.

Walkthrough

Frailties in the recycling and recollection arguments (0:00)

Kagan revisits the argument from recollection, suggesting an alternative explanation: encountering imperfect objects in the world might trigger our minds to grasp the forms for the first time, rather than reminding us of a prior acquaintance. He then shows that even granting Plato the recollection argument and combining it with the recycling argument, the conclusion only supports that the soul predates the body, not that it outlasts it. His car analogy makes the point: an engine is a pre-existing part of a car, but that does not mean the engine is indestructible or immortal.

The argument from simplicity (9:21)

Kagan builds the argument step by step using everyday examples: paper and chalk can be destroyed because they have parts, while the number three cannot be destroyed because it is simple, lacking parts. He generalizes this into premises: composite things can be destroyed, changing things are composite, and invisible things (like the forms) don't change. Putting these together yields that invisible things can't be destroyed, and since the soul is invisible, it can't be destroyed either. He also notes Socrates' odd hedge that the soul is "indestructible, or nearly so," and Cebes' objection that near-indestructibility (like a coat that outlasts its owner) is not the same as immortality.

Simmias' harmony objection (19:01)

Simmias challenges the premise that invisible things can't be destroyed by pointing to harmony: you cannot see harmony, but you can destroy it by smashing the instrument producing it. This matters because some thinkers propose the mind is like the harmony of the body, an early gesture at a physicalist view where the mind depends on the body functioning properly, much as harmony depends on a well-tuned harp.

Harmony as a counter analogy (30:36)

Kagan argues that even if Socrates could show harmony is a poor analogy for the mind, that would not rescue the argument, since Simmias only needs harmony to be an invisible, destructible thing to break the general premise. He then distinguishes three meanings of "invisible": unseeable, unsensable by any of the five senses, and undetectable altogether. Using a quotation from the text, he argues Socrates meant the second sense, which does let Socrates dismiss harmony as a counterexample, since harmony can be heard.

Radio waves as a new counterexample (42:36)

Kagan proposes radio waves as a better counterexample: they cannot be seen, touched, smelled, tasted, or heard, yet they can be destroyed. This forces a retreat to the third, strongest sense of invisible, meaning undetectable by any means, even indirect effects. But under that definition, Kagan argues the soul no longer qualifies as invisible, since we detect a soul's presence through its effects on the body, much as we detect a radio wave through a radio. He concludes the argument from simplicity ultimately fails.

Before you watch

• Review the earlier lecture on the argument from recollection, since this lecture assumes familiarity with it and the argument from recycling.
• Recall the basic distinction between dualism and physicalism about the mind, introduced earlier in the course.

Check your understanding

1. Why does combining the recycling and recollection arguments fail to prove the soul survives death, even if both are granted?
2. State the four premises of the argument from simplicity in your own words.
3. Why does harmony fail as a counterexample once "invisible" is defined as "cannot be sensed by any of the five senses"?
4. Why does the radio wave example still work as a counterexample under that same definition?
5. Once "invisible" is redefined as "undetectable by any means," why does Kagan think the soul no longer counts as invisible?