'The quantity of money as determined by the action of the central bank' is taken as given (i.e. exogenous - p. 247) and constant (because hoarding is ruled out on page 174 by the fact that the necessary expansion of the money supply cannot be 'determined by the public'). Keynes does not put a subscript 'w' on L or M, implying that we should think of them in money terms. This suggestion is reinforced by his wording on page 172 where he says "Unless we measure liquidity-preference in terms of wage-units (which is convenient in some contexts)... ". But seventy pages later there is a fairly clear statement that liquidity preference and the quantity of money are indeed "measured in terms of wage-units" (p246). The Keynesian economic system Keynes's economic model In Chapter 14 Keynes contrasts the classical theory of interest with his own, and in making the comparison he shows how his system can be applied to explain all the principal economic unknowns from the facts he takes as given. The two topics can be treated together because they are different ways of analysing the same equation. Keynes's presentation is informal. To make it more precise we will identify a set of 4 variables – saving, investment, the rate of interest, and the national income – and a parallel set of 4 equations which jointly determine them. The graph illustrates the reasoning. The red S lines are shown as increasing functions of r in obedience to classical theory; for Keynes they should be horizontal. The first equation asserts that the reigning rate of interest r̂ is determined from the amount of money in circulation M̂ through the liquidity preference function and the assumption that L(r̂)=M̂. The second equation fixes the level of investment Î given the rate of interest through the schedule of the marginal efficiency of capital as I(r̂). The third equation tells us that saving is equal to investment: S(Y)=Î. The final equation tells us that the income Ŷ is the value of Y corresponding to the implied level of saving. All this makes a satisfying theoretical system. Three comments can be made concerning the argument. Firstly, no use is made of the 'first postulate of classical economics', which can be called on later to set the price level. Secondly, Hicks (in 'Mr Keynes and the "Classics"') presents his version of Keynes's system with a single variable