Plato's Arguments for Soul

Sandra LaFave

1. The affinity argument (Phaedo 77d-80c).


See the text.

I got the following formulation of the affinity argument from Samuel C. Rickless' page here. Rickless teaches at UCSD.

Noncomposite things are indissoluble.
Things that always remain the same in the same state are most likely noncomposite.
Forms always remain the same in the same state.
Sensible things never remain the same in the same state.
Forms are invisible.
Sensible things are visible.
So, invisible things always remain the same in the same state, and visible things never remain the same in the same state.
The soul is invisible.
So the soul is indissoluble.

When the soul investigates things that remain the same in the same state, it too remains the same in the same state.
The soul investigates the Forms.
So, the soul is indissoluble.

The nature of the divine is to rule and lead.
The nature of the soul is to rule and lead (the body).
If X is F and Y is F, then X resembles Y.
So, the soul resembles the divine.
The divine is indissoluble.
If X resembles Y and Y is G, then X is G.
So the soul is indissoluble.



2. The "use" argument (Alcibiades I, 129B-130C)


A user differs from what is used.

A human uses his/her body.

Therefore a human must be different from the body.


A person is either body, soul, or a combination.

A person is not body (by the argument above).

A person is not a combination (by argument above)

Therefore, a person is soul.



3. The "proper work" argument in Republic 352D-354A


The proper work or function of a thing is that which it only does, or it does best.

Whatever operates does its work well by virtue, and badly by defect or vice.

The proper function of soul is reason, management, rule, deliberation, AND life (soul is the "vital principle").

Therefore the virtue of the soul is to reason, manage, rule, deliberate well.

Corollary: virtuous souls also exercise the other proper function of soul, namely, they LIVE (as long as they reason, manage, etc.)



4. The recollection argument in Meno 81B-E, 85B-86B


We find ourselves with knowledge that is unchanging (e.g., math).

World of the senses is always changing.

Therefore our knowledge of the unchanging can't come from the world of the senses.


Knowledge comes either from senses or it is innate (inborn).

Our knowledge of the unchanging doesn't come from the senses.

Therefore this knowledge must be innate.


Knowledge requires a knower.

Therefore, if innate knowledge exists, a knower of this innate knowledge must also exist.

A knower must exist in order to acquire knowledge.

Therefore, knowers of innate knowledge must exist before they are born, in order to acquire the knowledge they possess at birth.

It is reasonable to assume that if we lived before birth, we will continue to live after death.


Another version in Phaedo (74E-75E, 76D-E):

We know with certainty that things in the world of the senses are not perfect: e.g., we know beautiful things are not absolutely beautiful.

We could not have this certainty if we did not already know what absolute beauty is.

Therefore we must know absolute beauty innately.

Therefore we must have lived before birth.



5. The "cycle of opposites" argument


In nature we observe cycles of opposites. Hot things become cold, and then cold things can become hot; living things become dead, and then life emerges from dead things, etc. (like yin-yang).

Therefore it is reasonable to suppose that when we die, we will be transformed into something alive.



6. The argument why soul must be something, and not just a "harmony" (Phaedo 85D-86D, 91E-92C, 94D-94E)


There are too many relevant differences between soul and harmony:

    Soul pre-exists. (This is supposedly established by the previous arguments.) Harmony doesn't pre-exist.

    Unlike harmony, soul leads, moves, is active.

    Harmony can be greater or less, but not soul.

    Souls can be more or less virtuous or wicked, but these terms do not apply to harmony.

    If the soul were harmony, all souls would be equally good, which is absurd.

Therefore, the soul must be a substance, and "far more divine" than harmony.

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