Although he self-identified as an Austrian economist, Rothbard's methodology was at odds with that of many other Austrians. In 1956, Rothbard deprecated the views of Austrian economist Fritz Machlup, stating that Machlup was no praxeologist and calling him instead a "positivist" who failed to represent the views of Ludwig von Mises. Rothbard stated that in fact Machlup shared the opposing positivist view associated with economist Milton Friedman. Mises and Machlup had been colleagues in 1920s Vienna before each relocated to the United States and Mises later urged his American protege Israel Kirzner to pursue his PhD studies with Machlup at Johns Hopkins University.