The purpose of this thesis is not to present exhaustively the ethical doctrine of Plato, but to disclose one specific aspect in the development of his ethical thought. We shall attempt to show that Plato realized the necessity for a metaphysical foundation for his ethical theory. The search itself will begin in the early or Socratic dialogues, continue through such major works of the middle period as Phaedo and Symposium, and culminate in a definite resolution of the problem through the (...) doctrine of the Republic. The exposition will be developed in three stages: (1) an indication of the ways in which the need for a "metaphysical foundation of morals" is expressed or implied in the early dialogues; (2) an account of the development of Plato's metaphysical theory in dialogues of the middle period; and (3) the argument that this metaphysic was for Plato the completion and foundation of his ethical theory. (shrink)