I perfectly agree with those who object to the practice of some economists, simply to copy out what they believe is an economic argument from textbooks of pure mathematics or theoretical mechanics or physics, and I hope you will not interpret what I am to say in the sense of that practice. … We must not copy out actual arguments but we can learn from physics how to build up an exact argument. … Most important of all is the consideration that there are obviously a set of concepts and procedures which, although belonging not to the field of pure mathematics but to the field of more or less applied mathematics, one of so general a character as to be applicable to an indefinite number of different fields. The concepts of Potential or Friction or Inertia are of that kind. …

[Schumpeter] asked me: “Are you not reminded, dear colleague, of general equilibrium theory in economics when you read modern mathematical physics?” I doubt that I had the courage to admit that I had sadly neglected mathematical physics, and I surely did not dare tell him that I doubted his own knowledge of that area.