Once one has seen it, however, one must conclude that it is the cause of all that is correct and beautiful in anything, that it produces both light and its source in the visible realm, and that in the intelligible realm it controls and provides truth and understanding, so that anyone who is to act sensibly in private or public must see it.

Socrates describes the Form of the Good in Book 6, the ultimate object of knowledge. The Form of the Good is the source of all other Forms—the source of the entire intelligible realm, of intelligibility itself, and of our cognitive capacity to know. Though Socrates is not able to describe the Form of the Good explicitly, he attempts to give us a sense of it by comparing it to the sun. It is only when a man grasps the Form of the Good that he achieves the highest level of cognition, understanding. When a guardian takes this last step he is finally ready to become a philosopher-king.