It would, indeed, be more sensible to build houses and the like; but if there are political and practical difficulties in the way of this, the above would be better than nothing". Book IV discusses the inducement to invest, with the key ideas being presented in Chapter 11. The 'marginal efficiency of capital' is defined as the annual revenue which will be yielded by an extra increment of capital as a proportion of its cost. The 'schedule of the marginal efficiency of capital' is the function which, for any rate of interest r, gives us the level of investment which will take place if all opportunities are accepted whose return is at least r. By construction this depends on r alone and is a decreasing function of its argument; it is illustrated in the diagram, and we shall write it as I (r). This schedule is a characteristic of the current industrial process which Irving Fisher described as representing the 'investment opportunity side of interest theory'; and in fact the condition that it should equal S(Y,r) is the equation which determines the interest rate from income in classical theory. Keynes is seeking to reverse the direction of causality (and omitting r as an argument to S()). He interprets the schedule as expressing the demand for investment at any given value of r, giving it an alternative name: "We shall call this the investment demand-schedule..." (p136). He also refers to it as the 'demand curve for capital' (p178). For fixed industrial conditions, we conclude that 'the amount of investment... depends on the rate of interest' as John Hicks put it in 'Mr. Keynes and the "Classics"'. Keynes proposes two theories of liquidity preference (i.e. the demand for money): the first as a theory of interest in Chapter 13 and the second as a correction in Chapter 15. His arguments offer ample scope for criticism, but his final conclusion is that liquidity preference is a function mainly of income and the interest rate. The influence of income (which really represents a composite of income and wealth) is common ground with the classical tradition and is embodied in the Quantity Theory; the influence of interest had also been noted earlier, in particular by Frederick Lavington (see Hicks's Mr Keynes and the "Classics"). Thus Keynes's final conclusion may be acceptable to readers who question the arguments along the way. However he shows a persistent tendency to think in terms of the Chapter 13 theory while nominally accepting the Chapter 15 correction. Chapter 13 presents the first theory in rather metaphysical terms.