The Metaphysical Structure in Plato's Unwritten Doctrine

self-made chart based on Giovanni Reale's book " A History of Ancient Philosophy"

 

Indeed, within the theory of Ideas, we cannot simply accept without reason that sensations are manifold. The multiplicity of sensations must be explained by recourse to the Ideas. Yet the Ideas themselves are also multiple. Thus a further question arises: why are there many Ideas rather than just one?

You see, the theory of Ideas is only the portion of Plato’s doctrine that was written down. The answer must also be sought in his unwritten teachings: namely, a doctrine of dual principles. Every Idea can be derived from this twofold origin of “the One” and “the Indefinite Dyad.” The Indefinite Dyad means the infinitely small and the infinitely large, while the One is what imposes determination upon that indeterminate pair.

In this way, the duality of principles explains the abundance of sensation while avoiding the monism of the natural philosophers. The principle is not merely a single Idea.