Phaedo tells Echecrates about the conversation Socrates had with his disciples on the last day before his execution. The context of the dialogue calls for a discussion on the philosophy of death.
The philosopher desires death because it separates his soul from his body. This is because the philosopher’s mission, to contemplate Ideas like absolute greatness, absolute smallness, is hindered by the body. However, even though the philosopher desires death, it should not be by his own hand, for the same reason that it is unlawful to escape from prison.
The soul is immortal and the body is mortal. Proving that the body is mortal is not hard, but proving that the soul is immortal is difficult. We must first prove that the soul existed before birth and also that it will exist after death. The soul's existence before birth is sufficiently proved by the same methods employed in the Meno: one can observe that the soul learns subjects like geometry by remembrance of concepts it knew before birth but has now forgotten.
Proving the soul's existence after death is difficult, but if one accepts the existence of Ideas, it should be relatively straightforward. Opposite Ideas coexist but do not exist in the same thing. For example Simmias is greater than Socrates and smaller than Phaedo, but he is not by himself greater or smaller; rather, he is greater or smaller relative to others. This concept also applies to things that are inseparable from their attributes. If cold and heat are opposed, then fire, which is inseparable from heat, cannot coexist with cold. It then follows that if life and death are opposed, then the soul, which is inseparable from life, cannot coexist with death. Therefore, the soul is immortal.
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