Could this be Hayek (rather than Keynes)? Prices and Production p.127 (second revised ed. 1935) "...the task of monetary theory is a much wider one than is commonly assumed; that its task is nothing less than to cove a second time the whole field which is treated by pure theory under the assumption of barter, and to investigate what changes in the conclusion of pure theory are made necessary by the introduction of indirect exchange." John Berdell