Ludwig Josef Johann Wittgenstein (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. He was professor in philosophy at the University of Cambridge from 1939 until 1947. In his lifetime he published just one book review, one article, a children's dictionary, and the 75-page Tractatus Logico-Philosophicus (1921). In 1999 his posthumously published Philosophical Investigations (1953) was ranked 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". Bertrand Russell described him as "the most perfect example I have ever known of genius as traditionally conceived, passionate, profound, intense, and dominating."
His work is often divided between his early period, exemplified by the Tractatus, and latter period, articulated in the Philosophical Investigations. The early Wittgenstein was concerned with the relationship between propositions and the world, and believed that by providing an account of this relationship he had solved all philosophical problems; these problems arise, he thought, because the logic of language is not evident in our ordinary use of language. The later Wittgenstein rejected many of the conclusions of the Tractatus, arguing that the meaning of words is constituted by the function they perform within any given language-game. Wittgenstein's influence has been felt in nearly every field of the humanities and social sciences, yet there are widely diverging interpretations of his thought.