Friedrich Wilhelm Nietzsche (October 15, 1844 - August 25, 1900) (ˈfʁiːdʁɪç ˈvɪlhəlm ˈniːtsʃə) was a nineteenth-century German philologist and philosopher. He wrote critiques of religion, morality, contemporary culture, philosophy, and science, using a distinctive German language style and displaying a fondness for aphorism. Nietzsche's influence remains substantial within and beyond philosophy, notably in existentialism and postmodernism. His style, and radical questioning of the value and objectivity of truth, raise considerable problems of interpretation, generating an extensive secondary literature in both continental and analytic philosophy. Nonetheless, his key ideas include interpreting tragedy as an affirmation of life, an eternal recurrence that has become subject to numerous interpretations, a reversal of Platonism, and a repudiation of (especially 19th-century) Christianity.

Nietzsche began his career as a philologist before turning to philosophy. At the age of 24 he became Professor of Classical Philology at the University of Basel, but resigned in 1879 due to health problems, which would plague him for most of his life. In 1889 he exhibited symptoms of a serious mental illness, living out his remaining years in the care of his mother and sister until his death in 1900.