"Keynes's answer was that the rate of interest was determined in the financial markets, and there is no reason why it should necessarily gravitate to the requisite level. It was not a very convincing answer. This was the weakness Hicks would exploit. The demand function for investment (which Keynes called the marginal efficiency of capital schedule) was the Trojan horse that allowed the forces of his enemies to attack the very heart of Keynes's case for an under-employment equilibrium and hence for active government. It was one of Keynes's closest collaborators who solved the dilemma. From 1936 on, Joan Robinson had argued that if Keynes was right about the determination of employment, then orthodox theory must be wrong about the determination of prices. In the mid-1960s she at last found a way of sustaining her argument."