4- Argument from Causation through Forms
Objection: The soul pre-exists, but even if it continues after death, it might not be immortal; it might eventually wear out and perish.
A full response to this objection requires a full treatment of the causes of generation and destruction.
As previously discussed, we can assume the existence of such forms as absolute beauty, goodness, and magnitude (see Argument 2). A thing is beautiful because it take part in absolute beauty. It is by beauty that beautiful things are beautiful and the same goes for forms such as largeness and smallness.
From this perspective, it would be better to say that a tall man is taller by virtue of tallness than being tall by a head because a person cannot be taller by the same thing that makes someone shorter (or by something that is short).
We can then infer that no object can come into being except by participation in the reality peculiar to its appropriate universal; therefore, one plus one equals two, not by virtue of addition, but by participation in duality.
For example, Simmias is bigger than Socrates due to his height, and Socrates has the attribute of shortness relative to the height of Simmias.
Additionally, Simmias is smaller than Phaedo because Phaedo has the attribute of tallness in comparison with the shortness of Simmias.
Therefore Simmias is both short and tall, because his shortness is surpassed by the tallness of Phaedo and his tallness is asserted over the shortness of Socrates.
We can further infer that a form (tallness) cannot be both tall and short; tallness will not admit shortness; if shortness approaches, it must either withdraw or cease to exist.
However, Socrates can receive both shortness and tallness, but tallness itself cannot receive shortness and remain what it is.
And as seen from the first argument, opposite things come from opposite things; but the form of opposite itself can never become opposite to itself.
Therefore snow can never admit heat and remain snow; fire cannot admit cold. Moreover, the attribute of the form is applicable to both the form and the object possessing the form. For example Three is both three and odd, though not identical with oddness.
A given form does not admit its opposite and everything that possesses that form also does not admit its opposite.
For example, three (which is odd) cannot admit anything that is even and remain three. Three is always accompanied by the opposite of even (odd), even though not odd itself. Two is always accompanied by the opposite of odd (even). Fire is always accompanied by the opposite of cold (hot).
Consequently, double will not admit the form of odd; one and a half will not admit the form of whole.
Therefore it takes fire to make a body hot, not heat. (It takes an object possessing the form of heat and not the form of heat itself). Hence it takes fever to make a body diseased, not the form of disease itself.
We can then infer that it takes the soul to make the body alive and not the form of Life; and the opposite of life is death; therefore soul cannot admit its opposite, which is death; and that which does not admit death is immortal. Therefore the soul is immortal.
What is immortal is imperishable. God and the form of Life are immortal.
And so when death comes, the mortal part dies, and the immortal part escapes unharmed and indestructible and so we can infer that our souls will exist in the next world.
If the soul is immortal, then it demands care at all times. It takes nothing to the next world except the education and training it received in this life. Then it is of supreme importance in helping the newly deceased at the very beginning of their journey there.