Also in 1931, Hayek critiqued Keynes's Treatise on Money (1930) in his "Reflections on the pure theory of Mr. J. M. Keynes" and published his lectures at the LSE in book form as Prices and Production. Unemployment and idle resources are, for Keynes, caused by a lack of effective demand; for Hayek, they stem from a previous, unsustainable episode of easy money and artificially low interest rates. Keynes asked his friend Piero Sraffa to respond. Sraffa elaborated on the effect of inflation-induced "forced savings" on the capital sector and about the definition of a "natural" interest rate in a growing economy. Others who responded negatively to Hayek's work on the business cycle included John Hicks, Frank Knight, and Gunnar Myrdal. Kaldor later wrote that Hayek's Prices and Production had produced "a remarkable crop of critics" and that the total number of pages in British and American journals dedicated to the resulting debate "could rarely have been equalled in the economic controversies of the past."