CH. 7 THE MEANING OF SAVING AND INVESTMENT 85

created by the banking system. This, indeed, is thefundamental proposition of monetary theory.

Both these propositions follow merely from thefact that there cannot be a buyer without a selleror a seller without a buyer. Though an individualwhose transactions are small in relation to the marketcan safely neglect the fact that demand is not a one-sided transaction, it makes nonsense to neglect it whenwe come to aggregate demand. This is the vitaldifference between the theory of the economic behaviourof the aggregate and the theory of the behaviour ofthe individual unit, in which we assume that changes inthe individual’s own demand do not affect his income.