This paper discusses how Mark Blaug reversed his thinking about the historiography of economics, abandoning 'rational' for 'historical' reconstruction, and using an economics of scientific knowledge argument against Paul Samuelson and others that rational reconstructions of past ideas and theories in the "marketplace of ideas" were Pareto inefficient. Blaug's positive argument for historical reconstruction was built on the concept of "lost content" and his rejection of the end-state view of competition in favor of a process view. He used these ideas (...) to emphasize path dependency in the development of economic thinking, thereby advancing an evolutionary view of economics that has connections to a Lakatosian understanding of economic methodology. The paper argues that Blaug was essentially successful in criticizing the standard rational reconstructionist view of the history of economic thought in economics, and that this is borne out by the nature of the change in recent economics. (shrink)