Thales, Anaximander, and Anaximines made bold proposals about the ultimate constituents of reality, while Heraclitus insisted that there is an underlying order to the changing world. Parmenides of Elea formulated a powerful objection to all these proposals, while later Greek theorists (such as Anaxagoras and the atomist Democritus) attempted to answer that objection. In fifth-century Athens, Socrates insisted on the importance of the fundamental ethical question—“How shall I live?”—and his pupil, Plato, and Plato’s pupil, Aristotle, developed elaborate philosophical systems to explain the nature of reality, knowledge, and human happiness. After the death of Aristotle, in the Hellenistic period, Epicureans and Stoics developed and transformed that earlier tradition. We will study the major doctrines of all these thinkers. Part I will cover Plato and his predecessors. Part II will cover Aristotle and his successors.