Plato's Four Arguments for the Immortality of the Soul from the Phaedo part 3

3- Argument from Affinity (Body / Soul duality) (78b-84b)

At this point, a question is raised: we are sure the soul exists before birth, but how can we be sure it continues after death?

In responding, we first need to ask what sort of thing would be naturally dispersed after death.

A composite or natural object will decompose, an incomposite one will not. Incomposite things are constant and invariable, and so these can thus be considered as absolute realities. Absolute Equality or Beauty are constant and invariable whereas concrete instances of beauty or equality are inconstant and variable. Concrete objects are sensual and perceivable whereas constant entities are only perceivable by thought, hence they are invisible.

One can therefore infer that invisible things are invariable and visible things are variable.

Now human beings are part body and part soul and the body is visible whereas the soul is invisible.

When the soul uses the body for inquiry (via the senses), it is drawn to the variable natures and gets confused; but when it investigates by itself, it passes into the pure, eternal, immortal,  immovable, and absolute nature which is constant and invariable; this latter process is then a kindred nature with the soul.

Therefore the body is variable and the soul invariable; and it would be good then, to assume that the divine part, the soul, should govern, and the mortal part, the body, should serve.

Conclusion:

Body	Soul
Human	Divine
Mortal	Immortal
Unintelligible	Intelligible
Multiform	Uniform
Dissoluble	Indissoluble
Never Self-Consistent	Always Self-Consistent
Variable	Invariable

The philosopher, following reason and shunning pleasure and passions, frees himself from the imprisonment of the body and does not fear death.

Part 4