"The Cambridge critics of the mainstream explanation of interest theory were right: only under very special conditions can interest be viewed as the marginal product of capital. However, they were wrong in condemning the marginal approach altogether. As the pioneers of the marginal revolution knew full well, the real interest rate is first and foremost a reflection of the higher subjective valuation of present versus future consumption goods. Starting from this point - rather than the dubious equality with the marginal product of capital - the ethical defense of interest payments follows quite simply." -- Robert P. Murphy (2007) "Interest and the Marginal Product of Capital: A Critique of Samuelson", Journal of the History of Economic Thought, V. 29, N. 4: 453 - 464Murphy does not justify this conclusion. His article analyzes neither intertemporal subjective valuations nor ethics.
I also don't think Murphy really justifies his hostility to mathematics:
"I hope to convince the reader that typical mainstream neoclassicals left themselves open to their Cambridge critics ... because their fascination with mathematical models led many of them to forget the insights of earlier thinkers." -- Robert P. Murphy (2007).Murphy does argue well that forgetfulness is displayed. He doesn't show that this is because of mathematics. One can argue that a problem with mainstream economics is not mathematics as such. It is rather the inappropriate insistence on mathematics always and a refusal to use more sophisticated mathematics for theoretical clarification in economics.
I might as well point out that I agree with Murphy on some points, in addition to his refusal to accept the equality of the rate of interest and the marginal product of capital. For example, I think Böhm-Bawerk was insightful in his criticisms of the naive productivity theory of interest. I have read neither von Thünen nor Samuelson's defense of von Thünen against Böhm-Bawerk. Murphy's presentation of Samuelson leads me to think that Samuelson's defense lost. But I don't see the problem with Samuelson's defense being necessarily the assumption of a steady-state, as opposed to the assumption of a one good model.