Summary: Republic (page 5)

What is the Idea of the Good?

This expression refers to the famous Platonic doctrine of the Ideas. Let us summarise it briefly.

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 the eternal and absolute Beauty, in which many beautiful things participate—these possessing only a relative and particular beauty.

Similarly, 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 that can be accessed only through thought: the intelligible world, which is pure, eternal, and absolute. This is the reality that the Creator took as his model when he fashioned our sensible world—a mere imperfect reflection of the intelligible world.

Within the framework of this doctrine, thus summarised, 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, in order to see something, we need an intermediary between the object and the eye—namely, sunlight—so too does the Good illuminate all that can be known.

The Idea of the Good is to the intelligible world what the sun is to the visible world in relation to sight and objects:

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.

However, the Good is neither truth nor knowledge itself, just as the sun is not light itself. The Good surpasses these: 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 a famous passage from 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, etc.), apprehended by belief. These constitute the two types of objects in the sensible world.

At a higher level, in the intelligible world, we encounter mathematical objects, the objects of discursive knowledge. Finally, 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 so many images, rising from hypothesis to hypothesis towards a conclusion.

We know, in fact, that mathematicians make use of visible figures (by tracing a diagonal on the blackboard, for example), but they reason not about these sensible figures themselves, which they have just drawn, but about the originals—the Ideas—that these 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.

On the other hand, in order to reach the second part of the intelligible world—which leads to a principle that is beyond all hypothesis—the soul must, starting from a hypothesis and without relying on the images used in the first case, conduct its inquiry using Ideas alone, taken in themselves.

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

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