The Form of the Good
Plato discusses the Idea of the Good only in the Republic. There, it is introduced as the hierarchically supreme Idea, and the reason for the existence of all other Ideas.
In the Republic Socrates resists his interlocutors’ pleas to define the Good, suggesting instead an indirect approach through the three famous parables: The parable of the Sun, the parable of the Divided Line, and the parable of the Cave. The Sun, whose light is the reason sensible beings become visible, but also receive life, is an “offspring” of the good; in the universe of sensibles it holds a role similar to the Good in the universe of intelligible things. Thanks to the Idea of the Good, the soul conceives of intelligible beings and attributes to them their truth, but, furthermore, the Idea of the Good is the reason for the existence of the other Ideas.
In the parable of the Divided Line, the Idea of the Good is described as the “ἀρχὴ τοῦ παντός”, a first principle that exists independently of our views, a first principle that is “unhypothetical”, and will form the final step in the process of acquiring knowledge and in the hierarchy of what is. In the suggestive parable of the Cave, the Good is that which is finally revealed to the freed prisoners, blinding them with its radiance.
The Idea of the Good is therefore something more than the hierarchically supreme Idea. Plato seems to attribute to it the role of a first principle; it is placed above the Ideas and is somehow responsible for their existence. In the last phase of Platonic philosophy, in Plato’s so-called unwritten doctrines, that same role is held by the One.