Philosophy, study of the nature of being and thinking, and more specifically of the human experience. Traditionally, philosophers have focused on four main areas: (1) logic, or the study of the formal structure of truthful arguments; (2) metaphysics, or the study of the nature of “being” or ultimate reality; (3) epistimology, or the theory of knowledge; (4) ethics, or moral and political philosophy. The earliest known attempts to raise distinctively philosophical questions go back to the 7th century B.C., when the pre-Socratic Greek philosophers were active. Their intellectual heirs were Socrates, Plato, and Aristotole. Later ancient philosophies included epicureanism, stoicism, and neo-Platonism. Foremost among medieval philosphers in the West were St. Augustine and St. Thomas Aquinas. Modern philosophy, identified by the parallel development of rationalism and empiricism, began with René Descartes and culminated in the philosophy of Immanuel Kant. The idealism of G.F.W. Hegel and the positivism of August Compte were major forces in 19th-century philosophy, forming a basis for the philosophy of dialectical materialism espoused by Karl Marx. The philosophical orientations of most 20th-century philosophers have their roots in Marxism, Kantianism, logical positivism, pragmatism, phenomenology, or existentialism. Although at one time areas such as the natural sciences, psychology, sociology, logic, and mathematics were all considered to be within the domain of the philosophers, today's philosophers tend to concentrate on more specialized areas of inquiry, such as the philosophy of logic, the philosophy of science, and the philosophy of religion.