money is only the medium by which the exchange is effected.
 John Maynard Keynes, The General Theory of Employment, Interest and Money (1936). copy citation

Context

“The brief answer which Ricardo gave to the contentions of Malthus and Chalmers seems to have been accepted as sufficient by most later economists. 'Productions are always bought by productions or services; money is only the medium by which the exchange is effected. Hence the increased production being always accompanied by a correspondingly increased ability to get and consume, there is no possibility of Over-production' (Ricardo, Prin. of Pol. Econ. p. 362) .” source