The proponents of the real side as the prime mover in business cycles stressed either investment in physical capital and the Acceleration Principle or the failure of consumption to keep pace with rising output. Both the interest rate and entrepreneurial expectations were recognized as key factors in determining changes in the level of capital creation. Under-consumption business cycle theories faced directly the relationship of saving and investment. About the closest Haberler comes to Say’s Law is at this point:” it is clear that the social function of saving is to release resources from the production of goods for immediate consumption for the production of producers’ goods.” However, this is followed shortly by the statement that while the “opponents [of under-consumption theories] have shown the theoretical possibility of a smooth absorption of savings in new investment, they have not shown its necessity.” (Ibid, p. 126, emphasis in original) A special case of real causes of the cycle was variation in agricultural output. The relative decline of importance of agriculture in the advanced economies by 1936 had reduced interest in this approach. The role of Jevons in such special theories has already been mentioned.