SOCRATES: Then the same applies to the good. Unless someone can give an account of the form of the good, distinguishing it from everything else, and can survive all examination as if in a battle, striving to examine things [c] not in accordance with belief, but in accordance with being; and can journey through all that with his account still intact, you will say that he does not know the good itself or any other good whatsoever.
Readings in Ancient Greek Philosophy: From Thales to Aristotle (p. 364). Hackett Publishing Company, Inc.. Kindle Edition.
Of course Plato/Socrates utterly fails to give an "account of the form of the good," relying entirely on vague analogies.