This paper considers how Keynes and Sraffa in December 1931 disagreed on the definition of the concept of commodity rate of interest that was to appear in Sraffa�s review of Hayek�s Prices and Production. An analysis of that disagreement, which emerges from two letters sent by Keynes to Sraffa, also allows us to see that Sraffa�s 1932 definition and the definition adopted by Key…