  Shortly after Schlick arrived in Vienna, he was invited by the mathematician Hans Hahn to participate in a seminar on Principia Mathematica by Alfred North Whitehead and Bertrand Russell. Upon its conclusion, Schlick organized (at the request of his students, Herbert Feigl and Friedrich Waismann) organized an extra-curricular discussion group, which came to be called the ‘Schlick Circle’ and (eventually) the ‘Vienna Circle’. Their first reading was Tractatus Logico-Philosophicus, written by Russell’s student, Ludwig Wittgenstein (Wittgenstein 1961; Stadler 2001, Ch. 5). Soon, Schlick was writing Wittgenstein, seeking additional copies of his work, telling him about the study group in Vienna, and requesting personal meetings. After several failed attempts, Schlick finally arranged with Wittgenstein’s sister, Margarete Wittgenstein Stonborough, to visit him in early 1927 (McGuinness 1967, 14). The first clear evidence that Schlick had (at last!) obtained a copy of the Tractatus was in a letter to Einstein in June of 1927. (Schlick 1927a) The following month, Schlick effusively described the Tractatus as “the deepest” work of the new philosophy (Schlick 1927b). Over the next few years, Schlick and Wittgenstein met as time permitted, carrying on philosophical discussions ranging over a broad array of topics, from the idea of geometry as syntax, to verificationist and operationalist theories of meaning, topics in logic and mathematics, and even solipsism.