He was a disciple of Socrates and the teacher of Aristotle, and he founded the Academy in Athens. His system of thought had a profound influence on Christian theology and Western philosophy. His philosophical writings, which cover metaphysics, politics, and ethics, are presented in the form of dialogues, with Socrates as the principal speaker; they include the Symposium and the Phaedo. 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. Plato’s political theories appear in the Republic, in which he explored the nature and structure of a just society. He proposed a political system based on the division of the population into three classes, determined by education rather than birth or wealth: rulers, police and armed forces, and civilians.