Initially sympathetic to Wieser's democratic socialism, Hayek's economic thinking shifted away from socialism and toward the classical liberalism of Carl Menger after reading von Mises' book Socialism. It was sometime after reading Socialism that Hayek began attending von Mises' private seminars, joining several of his university friends, including Fritz Machlup, Alfred Schutz, Felix Kaufmann, and Gottfried Haberler, who were also participating in Hayek's own, more general, private seminar. It was during this time that he also encountered and befriended noted political philosopher Eric Voegelin, with whom he retained a long-standing relationship.