http://www.mises.org/books/robbinsessay2.pdf See footnote 2 p.69: The beginning of the change dates from the coming of the subjective theory of value. So long as the theory of value was expounded in terms of costs, it was possible to regard the subject-matter of Economics as something social and collective, and to discuss price relationships simply as market phenomena. With the realisation that these market phenomena were, in fact, dependent on the interplay of individual choice, and that the very social phenomena in terms of which they were explained---costs---were in the last analysis the reflex of individual choice---the valuation of alternative opportunities (Wieser, Davenport)---this approach becomes less and less convenient. The work of the mathematical economists in this respect only sets out particularly boldly a procedure which is really common to all modern theory. Cheers, Alan Isaac