In every dialogue apart from the Parmenides in which the Theory of Forms puts in an appearance, the main topic is something else. In the Phaedo it is immortality: the Forms are used in three arguments for the immortality of the soul. But they make a separate entrance before that.

64C: AN EXISTENTIAL ADMISSION

Socrates defends his readiness to die beginning with the question (64c2) “do we think there is such a thing as death?” (ἡγούμεθά τι τὸν θάνατον εἰ̃ναι;). He immediately says what death is (c4–5): “the release of the soul from the body” or the “coming-to-be in separation” of soul and body (c5–8). He now argues that the philosopher, whom he portrays as ascetic, is engaging in separating his soul from his body (64c–69e).

Socrates' question at 64c2 concerns an existential claim to which Simmias' assent commits him. It is, once again, a topic-fixer: Simmias is only conceding that people sometimes die; he is not committed by his concession to the Theory of Forms. It is a clear implication of 105d9, d13–14, etc., that there is a Form for death. But it is only in retrospect that we can say: in 64c, Simmias had already acknowledged the existence of the Form, Death; that is like saying that someone who has acknowledged the existence of the number 13 has committed himself to the existence of a prime number, or to the existence of a positive root of the equation “845 + 1183x ⊟ 96x2 = 0.”