Summary: Republic (page 5)

What is the Idea of the Good?

This expression refers to the famous Platonic doctrine of the Ideas. A brief summary is in order.

For Plato, things are beautiful because they partake in the Idea of Beauty itself; men are courageous insofar as they partake in the Idea of Courage itself, and so on.

To take the example of Beauty itself (already expounded in The Symposium): it is eternal and absolute Beauty, in which many beautiful things participate—these possessing only a relative and particular beauty.

Likewise, the Idea of Man itself represents what is common to all individual men.

Ideas are not to be found in the sensible world, which is imperfect and subject to decay, but in another reality accessible only through thought: the intelligible world, which is pure, eternal, and absolute. This is the reality the Creator took as his model when fashioning our sensible world—a mere imperfect reflection of the intelligible.

Within this framework, how does Plato define the precise Idea of the Good itself in The Republic?

It is that which is present in the multiplicity of beautiful and good things.

Just as seeing requires an intermediary between the eye and its object—namely, sunlight—so too does the Good illuminate all that can be known.

The Idea of the Good stands in relation to the intelligible world as the sun does to the visible world:

That which sheds the light of truth on the objects of knowledge and confers on the knowing subject the power to know is the Idea of the Good ¹.

Is it then beyond all knowledge? On the contrary: Since [the Idea of the Good] is the principle of knowledge and truth, you can conceive of it as an object of knowledge ².

Yet the Good is neither truth nor knowledge itself, just as the sun is not light itself. The Good surpasses both: The nature of the Good must be regarded as far more precious ³.

This Idea reigns supreme in the intelligible world: Intelligible things derive their intelligibility, their being, and their essence from the Good ⁴.

Plato then establishes a hierarchy of beings according to their degree of clarity, self-evidence, and truth, in one of the most celebrated passages of The Republic.

At the lowest level are images of things—their shadows and reflections—apprehended by the imagination. Next come the things themselves (animals, plants, works of art, and so on), apprehended by belief. These make up the two types of objects in the sensible world.

Higher up, in the intelligible world, we encounter mathematical objects, the objects of discursive knowledge. At the supreme level—where being and knowledge converge—are the Ideas, which Plato also calls Forms, apprehended by pure intelligence.

To reach the first part of the intelligible world—the mathematical domain—the soul uses the originals of the visible world as images, moving from hypothesis to hypothesis towards a conclusion.

Mathematicians do make use of visible figures (tracing a diagonal on a blackboard, for example), but they reason not about these sensible figures themselves, but about the originals—the Ideas—that such figures reproduce:

Their reasoning relates to the diagonal in itself, not to the diagonal they draw. As for the figures they trace, which have their reflections in water, they use them as so many images in an attempt to grasp those things in themselves, which we otherwise perceive only through thought ⁵.

To reach the second part of the intelligible world—which leads to a principle beyond all hypothesis—the soul must set out from a hypothesis and, without relying on visible images, conduct its inquiry using Ideas alone, taken in themselves.

The science used to reach this higher part of the intelligible world is, for Plato, no longer mathematics but dialectics.

¹ 508b-509a
² Ibid.
³ Ibid.
⁴ 509a, 510a
⁵ 510a-511b