Ludwig Wittgenstein is one of the most influential philosophers of the twentieth century, and regarded by some as the most important since Immanuel Kant. His early work was influenced by that of Arthur Schopenhauer and, especially, by his teacher Bertrand Russell and by Gottlob Frege, who became something of a friend. This work culminated in the Tractatus Logico-Philosophicus, the only philosophy book that Wittgenstein published during his lifetime. It claimed to solve all the major problems of philosophy and was held in especially high esteem by the anti-metaphysical logical positivists. The Tractatus is based on the idea that philosophical problems arise from misunderstandings of the logic of language, and it tries to show what this logic is. Wittgenstein’s later work, principally his Philosophical Investigations, shares this concern with logic and language, but takes a different, less technical, approach to philosophical problems. This book helped to inspire so-called ordinary language philosophy. This style of doing philosophy has fallen somewhat out of favor, but Wittgenstein’s work on rule-following and private language is still considered important, and his later philosophy is influential in a growing number of fields outside philosophy.