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2.11 Keynes and the Quantity Theory of Money

In the classical model a monetary impulse has no real effects on the economy.

Money is neutral. Since the quantity of real output is predetermined by the

combined impact of a competitive labour market and Say’s Law, any change

in the quantity of money can only affect the general price level. By rejecting

both Say’s Law and the classical model of the labour market, Keynes’s theory

no longer assumes that real output is predetermined at its full employment

level. In Chapter 21 of the General Theory, Keynes discusses the various

possibilities. If the aggregate supply curve is perfectly elastic, then a change

in effective demand brought about by an increase in the quantity of money

will cause output and employment to increase with no effect on the price

level until full employment is reached. However, in the normal course of

events, an increase in effective demand will ‘spend itself partly in increasing

the quantity of employment and partly in raising the level of prices’ (Keynes,

1936, p. 296). In other words, the supply response of the economy in Keynes’s

model can be represented by an aggregate supply function such as W0AS in

Figure 2.6, panel (b). Therefore for monetary expansions carried out when Y

< YF, both output and the price level will rise. Once the aggregate volume of

output corresponding to full employment is established, Keynes accepted that

‘the classical theory comes into its own again’ and monetary expansions will

produce ‘true inflation’ (Keynes, 1936, pp. 378, 303). A further complication

in Keynes’s model is that the linkage between a change in the quantity of

money and a change in effective demand is indirect, coming as it does via its

influence on interest rates, investment and the size of the multiplier. We

should also note that, once Keynes had introduced the theory of liquidity

preference, the possibility that the demand for money function might shift

about unpredictably, causing velocity to vary, implies that changes in M may

be offset by changes in V in the opposite direction. With Y and V no longer

assumed constant in the equation MV = PY, it is clear that changes in the

quantity of money may cause V, P or Y to vary. The neutrality of money is no

longer guaranteed.