Book 6: The Sun and the Divided Line

Adeimantus objects: actual philosophers seem useless or, worse, wicked. Look at the philosophers around Athens. Why would we put any of them in charge of a city? Socrates answers with the ship-of-state metaphor (488a-489d). The shipowner is large but slightly deaf and short-sighted. The sailors quarrel over the rudder, each claiming the right to steer though none have studied navigation. The one sailor who has studied the stars and the wind is dismissed by the others as a useless dreamer. He is in fact the only one who can pilot the ship. The philosopher is corrupted by the city's misunderstanding of him, not by anything in philosophy itself.

Genuine philosophers are rare because the philosophic nature requires a specific set of traits — good memory, ease of learning, magnanimity, love of truth — and those same traits make the person attractive to flatterers and dangerous to corrupt regimes. Most philosophic natures are destroyed by their environment before they ripen. The few that survive are usually solitary, holed up in private study, refusing the public role they could fill.

The philosopher's education must culminate in vision of the Form of the Good. The Good is what gives the Forms their intelligibility, as the sun gives visible objects their visibility. This is the Sun Analogy (508a-509b). Without sunlight, even open eyes cannot see; without the Good, even the trained intellect cannot truly know. The Good is "beyond being" — the source of being for all the Forms but not itself one Form among others.

The Divided Line (509d-511e) maps the four epistemic levels. Take a line and cut it into two unequal segments; cut each segment again in the same ratio. The four resulting segments correspond to: (1) eikasia — perception of images, shadows, reflections; (2) pistis — belief about physical objects; (3) dianoia — reasoning about mathematical objects using diagrams as starting points; (4) noēsis — direct intellectual grasp of the Forms via dialectic, with no diagram needed. Each segment is also a stage of education; the philosopher ascends from one to the next.

The destination is direct insight into the Form of the Good — the source of intelligibility itself. Plato is making an extraordinary claim: that there is a knowable structure to reality, that the structure is mathematical-philosophical rather than physical, and that the human intellect properly trained can apprehend it. Whitehead's later phrase that all Western philosophy is footnotes to Plato traces back to claims like this one. The Sun Analogy and the Divided Line set up Book 7's allegory.