Keynes’s great contribution to monetary theory, and to the policies of his time, was based on his refutation of an important element of classical economic theory. He argued that the rate of interest was the cause, not as orthodox economists argued the passive consequence, of the level of economic activity. In other words, the level of investment, employment, and trade was caused by the rate of interest. If the rate was too high, the level of investment, employment and trade would fall. If it was low, the level of investment, employment and trade would rise.