Initially the term referred to any body of knowledge. In this sense, philosophy is closely related to religion, mathematics, natural science, education, and politics. Though it has since been classified as a book of physics, Newton’s Mathematical Principles of Natural Philosophy.
In the 19th century, the growth of modern research universities led academic philosophy and other disciplines to professionalize and specialize. Since then, various areas of investigation that were traditionally part of philosophy have become separate academic disciplines, such as psychology.