Western philosophy began in Ancient Greece, dealing with the basic nature of the world, known as ontology; the discussion of whether it's possible to know any fact for certain, called epistemology; and the basis for moral judgment, or ethics. These subjects are still studied in academic courses on philosophy as of 2015. An overview of philosophical thought can be found at Philosophy Pages.
Philosophy is one of the oldest fields of human study. One of its early branches, nature philosophy, is the forerunner of scientific investigation. During the Middle Ages, philosophical debates were stifled in Europe and largely limited to theological issues. The Greek philosophical classics were preserved and elaborated upon in the Islamic world and reintroduced to Europe with the Renaissance.
With the advance of scientific thought, philosophy of science appeared as a distinct branch of the field in the 17th and 18th centuries. Political philosophy became an important branch beginning with emergence of Marxism in the 19th century. In the 20th and 21th century, the study of language and its relation to reality has been an important field.
Some of the most influential philosophers through history are Socrates, Aristotle, Kant and Wittgenstein. Largely independent from the Western tradition up to Modern times, Asian philosophers have pondered many of the same problems, beginning in India at roughly the same time as in Greece.