Phaedo

The dialogue's deeper achievement is the introduction of the 'new method of hypothesis' (99d–102a). When Socrates abandons the search for physical causes — why Socrates sits in prison, why he is in Athens — he turns to a method of positing Forms as the only genuine explanations. The proof of immortality is explicitly a means for establishing that the Ideas are the sole causes and objects of knowledge. The dialogue thus establishes the Theory of Forms as the foundation of philosophy, with immortality as its necessary consequence, rather than the reverse.

Simmias, drawing on his Pythagorean background, proposes that the soul may be a harmonia — an attunement of the body's opposing elements — rather than a separate substance. If true, the soul would be invisible and divine in a sense, but would perish when the body's tension is broken, just as the harmony of a lyre vanishes when the instrument is destroyed. This objection attacks the Argument from Affinity by showing that invisibility alone does not guarantee immortality. Socrates must distinguish the soul as that which rules the body from a mere harmony that follows it.

The dialogue's arguments have provoked doubt from the start. Cicero's interlocutors report agreeing while reading but losing conviction once the book is set aside. The Argument from Opposites, in particular, has been criticized as a hazardous leap from biological cycles to metaphysical claims. Modern commentators note that the final proof depends on the Theory of Forms and the exclusion of opposites — premises many readers do not accept. Yet the dialogue's enduring value lies less in its conclusions than in its method: Socrates' insistence that arguments be examined and re-examined, and his warning against misology when they fail.

The Phaedo contains Plato's first sustained exposition of the Theory of Forms in the middle dialogues. The Theory of Recollection presupposes that we possess knowledge of absolute Equality, Beauty, and Goodness acquired before birth. The final proof of immortality depends on the claim that Forms cannot admit their opposites — the Form of Life excludes death, so the soul, which brings life, cannot perish. The dialogue thus weaves epistemology, metaphysics, and psychology into a single argument, establishing Forms as both the objects of knowledge and the ground of the soul's immortality.

The closing myth (107c–115a) describes the true earth, the underworld rivers, and the destiny of purified and unpurified souls. It does not function as a proof but as a persuasive image, a 'noble risk' Socrates acknowledges. The myth translates the abstract arguments into a vivid picture of the philosophical life's stakes: the soul's fate depends on how it has lived. It also introduces the doctrine of transmigration, with souls assuming different forms according to their conduct. The myth's truth is presented as uncertain, but its ethical function is clear.

Socrates argues that death is the separation of soul from body, and that the philosopher's whole life is a training in that separation. The body distracts the soul with senses, desires, and fears; genuine knowledge of the Forms requires turning away from bodily concerns toward pure thought. The philosopher therefore does not fear death but welcomes it as the completion of what he has practiced all along. This claim, introduced early in the dialogue, frames the entire discussion: the proofs of immortality show that this practice is rational, not merely wishful.