Knowledge of values, in fact, is a matter of direct insight, like seeing that the sky is blue, the grass green. It does not consist of pieces of information that can be handed from one mind to another. In the last resort, every individual must see and judge for himself what it is good for him to do. […] A man perfect in self-knowledge can tell when his own vision of what is good is clear; he cannot see into another’s mind and tell whether his vision is clear.
—F.M. Cornford (Before and After Socrates, p. 46f.)