This paper goes back to a conversation about the methodology of economics in which L. Mises claimed that certain propositions of economics can be proved. As an example, he mentioned the law of diminishing returns and referred me to the literature for the proofs. After I presented my findings to the Viennese Economic Society they were published in [1936, 6] and [1936, 6a]. In 1936, Mises wrote me that he learned a great deal from the paper. An English translation appeared in [1954, 3]. In what follows, that translation has been thoroughly revised and shortened and the mathematical parts have been relegated to paragraphs in small print.