The discussion on ‘is’ and ‘ought’ in economics came to Walras’s attention through the work of J.-B.Say and his students.2 Whereas Say, according to Walras, had defined political economy as a natural science, his students proposed to modify this definition by introducing elements of both a natural science and a moral science into economics. It was proposed that the combination of a natural and a moral science should form a distinction between, and a synthesis of, the science and the art of political economy. Thus, the science of political economy would concern itself with the observation and description of real phenomena from which rules would be deduced to form the basis of the art.3 The term ‘art’ also appeared in Walras’s work, though carrying a different meaning. In Walras’s work, art related to that body of

knowledge prescribing rules of human conduct in the organization of production.