Ludwig Josef Johann Wittgenstein (; German: [ˈluːtvɪç ˈvɪtgənˌʃtaɪn]; 26 April 1889 – 29 April 1951) was an Austrian philosopher who worked primarily in logic, the philosophy of mathematics, the philosophy of mind, and the philosophy of language.

From 1929 to 1947, Wittgenstein taught at the University of Cambridge. During his lifetime he published just one slim book (the 75-page Tractatus Logico-Philosophicus, 1921), one article ("Some Remarks on Logical Form", 1929), one book review and a children's dictionary. His voluminous manuscripts were edited and published posthumously. Philosophical Investigations appeared as a book in 1953, and has since come to be recognised as one of the most important works of philosophy in the 20th century. His teacher, Bertrand Russell, described Wittgenstein as "perhaps the most perfect example I have ever known of genius as traditionally conceived; passionate, profound, intense, and dominating".

Born in Vienna into one of Europe's richest families, he inherited a fortune from his father in 1913. He initially made some donations to artists and writers, and then, in a period of severe personal depression after the First World War, he gave away his entire fortune to his brothers and sisters. Three of his four brothers committed suicide, which Wittgenstein had also contemplated. He left academia several times—serving as an officer on the front line during World War I, where he was decorated a number of times for his courage; teaching in schools in remote Austrian villages where he encountered controversy for hitting children when they made mistakes in mathematics; and working as a hospital porter during World War II in London, where he told patients not to take the drugs they were prescribed while largely managing to keep secret the fact that he was one of the world's most famous philosophers. He described philosophy as "the only work that gives me real satisfaction".

His philosophy is often divided into an early period, exemplified by the Tractatus, and a later period, articulated in the Philosophical Investigations. "Early Wittgenstein" was concerned with the logical relationship between propositions and the world and he believed that by providing an account of the logic underlying this relationship, he had solved all philosophical problems. "Late Wittgenstein," however, rejected many of the assumptions of the Tractatus, arguing that the meaning of words is best understood as their use within a given language-game. His mentor Bertrand Russell was likely the first to coin this distinction in Wittgenstein's work.

A survey among American university and college teachers ranked the Investigations as the most important book of 20th-century philosophy, standing out as "the one crossover masterpiece in twentieth-century philosophy, appealing across diverse specializations and philosophical orientations." The Investigations also ranked 54th on a list of most influential twentieth-century works in cognitive science prepared by the University of Minnesota's Center for Cognitive Sciences. However, in the words of his friend Georg Henrik von Wright, he believed "his ideas were generally misunderstood and distorted even by those who professed to be his disciples. He doubted he would be better understood in the future. He once said he felt as though he was writing for people who would think in a different way, breathe a different air of life, from that of present-day men."