A disciple of Socrates and the teacher of Aristotle, he founded the Academy in Athens. An integral part of his thought is the theory of 'ideas'or 'forms', in which abstract entities or universals are contrasted with their objects or particulars in the material world. His philosophical writings are presented in the form of dialogues, with Socrates as the principal speaker; they include the Symposium and the Timaeus. Plato's political theories appear in the Republic, in which he explored the nature and structure of a just society.