The branches of philosophy (red) and their links with allied sciences (yellow).
Philosophy (from philosophos, lover of wisdom) is any body of doctrine or opinion as to the nature and ultimate significance of human experience considered as a whole. It is perhaps more properly applied to the critical evaluation of all claims to knowledge – including its own and anything that is presupposed about its own nature and task. In this latter respect, it is widely argued, philosophy differs fundamentally from all other disciplines. What philosophy "is" (what methods philosophers should employ, what criteria they should appeal to, and what goals they should set themselves) is as perennial a question for the philosopher as any other.
Traditionally, philosophers have concerned themselves with four main topic areas: logic, the study of the formal structure of valid arguments; metaphysics, usually identified with ontology – the study of the nature of "Being" or ultimate reality; epistemology, or theory of knowledge, sometimes treated as a branch of metaphysics, and axiology, or theory of value – including aesthetics, the philosophy of taste (especially as applied to the arts), ethics, or moral philosophy, and political philosophy or political science. In modern times as traditional philosophy has yielded up the subject matters of the natural sciences, of other descriptive studies such as psychology and sociology, and of such formal studies as logic and mathematics, all once numbered among its legitimate concerns, philosophers have become increasingly conscious of their critical role. Most now tend to interest themselves in special philosophies, e.g., the philosophy of logic, the philosophy of science, and the philosophy of religion.
The first attempts to answer distinctively philosophical questions were made from about 600 BC by certain Greek philosophers known collectively as the Presocratics (see pre-Socratic philosophy); their intellectual heirs were Socrates, Plato, and Aristotle, the three towering figures in ancient philosophy. Later ancient philosophies included epicureanism, stoicism, and neoplatonism. Foremost among medieval philosophers were St. Augustine and Thomas Aquinas, both leading churchmen.
Modern philosophy begins with René Descartes and a parallel development of rationalism and empiricism culminating in the philosophy of Immanuel Kant. The idealism of Georg Hegel and the positivism of Auguste Comte were major forces in the 19th-century philosophy. The dialectical materialism of Karl Marx had its roots in both. (See also materialism.) The philosophical orientations of most twentieth-century philosophers were developments of Marxism, neokantianism, logical positivism, pragmatism, phenomenology, and existentialism.