This paper argues that Robbins famous definition of economics was of economic science which he saw as only a narrow branch of the field of economics. The field of economics included both economic sciencewhich his definition dealt with, and political economy--which his essay did not deal with. His prescriptive message was that policy belonged in the political economy branch of economics. He believed that while the science of economics should avoid value judgments as much as possible, the political economy (applied policy) branch of economics should, and must, include value judgments. That prescriptive message has been lost.