Twentieth-century philosopher whose two major works, Tractatus Logico-Philosophicus and Philosophical Investigations determined the course of Anglo-American philosophy for a long time after their publication. Each book was the impetus for a school of philosophy; however, Wittgenstein himself believed that both groups (and just about everybody else) misunderstood his writings.

The Tractatus was the product of several years of work trying to solve problems in logic and the foundations of mathematics. However, as Wittgenstein came closer to what he perceived as a solution (the preface to the work claims that all philosophical problems have been solved), his interests in philosophy broadened. By the time he wrote the draft of the Tractatus that was eventually published, he thought of it as a work in ethics. He wrote to a friend that the most important part of the book was that which was not written. This is not how the Vienna Circle (composed of Moritz Schlick, Friedrich Waismann, Rudolf Carnap, Otto Neurath, and others) viewed it. To these logical positivists, the Tractatus was a perfect explication of their belief in the principle of verificationism.

Philosophical Investigations was not published during Wittgenstein's lifetime, but drafts of sections (particularly the Blue Book) were circulated. In this work, Wittgenstein introduced the notion of a language-game. The language used in this work is deceptively simple, and the book is written in a more approachable style than the Tractatus. It often seems that Wittgenstein uses the way people talk in everyday language to provide support for his claims. Whether or not this was his intention, this is how the work was perceived by a group of philosophers (including J. L. Austin, Gilbert Ryle, and Peter Strawson) who became known as the ordinary-language philosophers.

Wittgenstein and Adolf Hitler did indeed attend the same school for a year. There is as far as I know no good evidence that they knew each other.