Now Plato goes a step further in his analysis. Not only is hot the contrary of cold, but there are some things that are necessarily cold, for instance snow. Snow is a form of frozen water; if you warm it up, it melts. Thus, not only does cold not allow the approach of heat, but snow also is incompatible with heat. There are, it turns out, necessary connections between some Forms and others, such as snow and cold. These Forms have necessary or essential connections with other Forms, without which things could not participate in the Forms. Snow can exist only in cold conditions, so that Cold is an essential concomitant of Snow.[18]
Plato now makes use of this kind of connectedness among Forms to address the outstanding problem for immortality of soul. Cebes has raised the possibility of a soul surviving a number of incarnations, but then perishing after one of these, like a tailor who did not outlive the last suit of clothes he made. Plato now points out that beside the “safe answer” he gave of what makes something F, namely F-ness, we can glimpse a new answer. He gives an example: not only can we say that Sickness makes us sick, but we can say that Fever, which is a kind of necessary concomitant of sickness, also makes us sick.
What does soul bring with it, to whatever it occupies? Life. It appears, then, that Life is a necessary concomitant of soul. And Life has an opposite, namely, Death. What does not admit of Death is deathless. And what is deathless is indestructible. So it appears that soul is indestructible. It seems to follow, then, that soul is immortal, with all that this implies.[19] When the soul leaves the body, the body dies; the body admits of death. But the soul itself remains alive by its very nature. Simmias concedes the point. Socrates observes that more discussion might be needed to convince everyone, but the basic argument seems sound and irrefutable.[20]
We might note that we have in this story the beginnings of logic. If some object x is F and F entails G, the x is G. If this snowball is snow, and snow is cold, then this snowball is cold. If Socrates has a soul, and soul has life, then Socrates has life.
[18] Plato Phaedo 103c-e.
[19] Plato Phaedo 105b-107a.
[20] Plato Phaedo 107a-b.