This paper investigates Lionel Robbins’s use of Max Weber’s criticism of psychophysics in managing the boundaries between economics, history, and psychology in his Essay. Max Weber’s criticism of psychophysics hinged on the notion of means-ends rationality. The logical structure of means-ends rationality made experimental and statistical methods of investigation redundant, turning its study into an analytical, rather than empirical and, for Weber, historical exercise. Robbins wholeheartedly accepted Weber’s criticism of psychophysics, but rejected Weber’s historicizing of the importance of means-ends rationality. Thus, Robbins turned the subject matter of economics into the a-historic study of constrained optimizing behavior. We see the irony of this position in contemporary neuro-economists’ acceptance of Robbins’s definition of economics to study the relation of brain-processes to behavior, while Robbins aimed to steer away from an investigation of man’s physiology in the service of economics.