In Phaedo 86e–88b, Cebes objects that none of the previous arguments for immortality has ruled out the possibility of the soul's wearing out: it might exist before we come-to-be, outlast a number of incarnations, and yet perish in the end. What needs to be shown is “that {the} soul is altogether deathless and imperishable” (88b5–6).

Socrates gets around to this objection in 95a–d, and coming-to-be, perishing, and their causes become the center of his attention. He introduces a theory about these matters, which turns out to be the Theory of Forms again. We are not here primarily concerned with the immortality of the soul. But, just as Socrates and Simmias earlier (76d7–77a5) agreed that the existence of the Forms and the preexistence of the soul somehow went together, so here Socrates is going to be telling us (100b7–9) that the existence of the Forms carries immortality with it. This claim, like the earlier one, can only be understood as highly elliptical.

Socrates does not propound his theory straight off. He first discusses some “mechanistic” (as I'll call them) theories he once held (95e–97b). He rejects all of them, and then (97b–99d) introduces a different sort of theory that we may call “teleological”; this, he seems to say, is the only sort of theory that could ever be really adequate, but he has so far been unable to formulate a satisfactory one. I discuss teleological theories in § 13.1.