Ludwig Wittgenstein, (1889-1951). Austrian philosopher. His interest in mathematics developed from his early researches in aeronautics. From 1911 he studied logic under Bertrand Russell at Cambridge. He fought in the Austrian army during World War I and the manuscript of his Tractatus Logico-Philosophicus (1921) was sent to Russell from an Italian prison camp. This complex and influential treatise explores the relationship between language and things. On his release in 1919, Wittgenstein gave up philosophy to work as a teacher until 1929, when he returned to Cambridge as a lecturer and, later, professor. Philosophical Investigations (1953) was published after his death from cancer.