Ludwig Josef Johann Wittgenstein (26 April 1889 – 29 April 1951) was an Austrian-British 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 by the Baruch Poll, standing out as "...the one crossover masterpiece in twentieth-century philosophy, appealing across diverse specializations and philosophical orientations". Philosopher Bertrand Russell described him as "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 wealthiest families, he gave away his entire inheritance. Three of his brothers committed suicide, with Ludwig contemplating it too. He left academia several times: serving as an officer on the frontline 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 during World War II as a hospital porter in London, where he told patients not to take the drugs they were prescribed, and where no-one knew he was one of the world's most famous philosophers. He described philosophy, however, as "the only work that gives me real satisfaction."
His philosophy is often divided between his early period, exemplified by the Tractatus, and later period, articulated in the Philosophical Investigations. The early Wittgenstein was concerned with the logical relationship between propositions and the world, and believed that by providing an account of the logic underlying this relationship he had solved all philosophical problems. 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. In the words of his friend and colleague Georg Henrik von Wright: "He was of the opinion... that 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 were writing for people who would think in a different way, breathe a different air of life, from that of present-day men."