Say's law
Keynes' innovation in this regard was twofold: First, he was to turn the mechanism that regulates savings and investment, the rate of interest, into a shell of its former self (relegating it to the "price of money") by showing that supply and investment were not independent of one another and thus could not be related uniquely in terms of the balancing of disutility and utility. Second, after Say's law was dealt with and shown to be theoretically inconsistent, there was a gap to be filled. If Say's law was the logic by which we thought financial markets came to a unique position in the long run, and if Say's law were to be discarded, what were the real "rules of the game" of the financial markets? How did they function and remain stable?