Ludwig Joseph Johann Wittgenstein (1889–1951), one of the most influential philosophers of the twentieth century. Austrian born, he abandoned a career in engineering to study philosophy of mathematics with BR at Cambridge in 1911 and started making original contributions, in the form of cryptic, posthumously published notes, shortly thereafter. In 1913 he criticized BR’s multiple-relation theory of judgment so effectively that BR abandoned the book (Theory of Knowledge) presenting the theory. During the First World War he served in the Austrian Army and completed the Tractatus Logico-Philosophicus (published in German as Logisch-philosophische Abhandlung in 1921 and in English translation under the title by which it became known in 1922), the only major work he published in his lifetime. He then abandoned philosophy for some years before returning to Cambridge in 1929, where he became a Research Fellow and began lecturing. He succeeded G.E. Moore as Knightbridge Professor of Philosophy in 1939. During this later period his philosophy took a very different direction from the one found in the Tractatus. He published nothing but wrote copiously; his notes, lectures, and remarks were posthumously published by his students and disciples in various editions and compilations, the most important of which was Philosophical Investigations (1953). Main biography: Ray Monk, Ludwig Wittgenstein: the Duty of Genius (London: Cape, 1990).