Tyler Cowen writes:
Keynes's General Theory, chapter six: in part ii the bombshell comes, unannounced. Keynes decides that he will declare savings to be a "mere residual." Consumption and investment alone will determine income and savings is defined as whatever is left over to make the national income equations balance. At the time this was considered by many to be an enormous sleight of hand. The Austrian and Swedish traditions focused on the question of whether planned savings was going to equal planned investment and what happens if not. Keynes has just banished such questions to the woodshed and he has done so by a terminological maneuver.
Whether or not you think that the Austrian and Swedish traditions lead anywhere fruitful, Keynes is on shaky ground here. He is using definitions to favor one causal account of macro over another. That's not right. You can still make a plausible argument that Keynes is right on empirical grounds that planned savings is not an important force for understanding business cycles. But so far no such empirical argument has been clinched...
I think it is much more than a terminological maneuver. Walras's law tells us that if one market is out of supply-demand balance, there must be another related market (or markets) that is also out of balance. If planned saving is in excess of planned investment, then planned consumption spending must be less than planned production of consumer goods. You can then follow the inventory adjustment chain--say that as inventories pile up producers cut back on the making of consumption goods. You then try to follow through on what is happening in the money market and you are led to the conclusion that ex ante savings must be destroyed by a process of deleveraging and deflation and... you wind up in the swamp. But you can be rescued from the swamp by recalling Walras's law, and recognizing that if you just follow the process by which equilibrium is restored in the goods market you will then discover that that process has also restored equilibrium in the flow-of-funds through financial markets.
Keynes's "terminological maneuver" would not have succeeded if it were not for the fact that Keynes's theory worked at a level that Wicksell's or Myrdal's or Ohlin's never could.