nous

In Book VI of the Republic we begin to discover the deeper metaphysics of Plato, where geometry and mathematics provide the means by which the truth of the higher realms can be seen by analogy.  Socrates engages in a bitter critique of the sophists and their pseudo-philosophy of crowd-pleasing and beastly, herd-like nonsense, lacking any real philosophical education.  Rather, the true philosopher is spoken of as grasping the Noetic Light of the Good, the true Sun of the Intelligible realm.