78 THE GENERAL THEORY OF EMPLOYMENT BK. II

investment over saving was the motive force governingchanges in the volume of output. Thus the new argu-ment, though (as I now think) much more accurateand instructive, is essentially a development of the old.Expressed in the language of my Treatise on Money , itwould run: the expectation of an increased excess ofInvestment over Saving, given the former volume ofemployment and output, will induce entrepreneurs toincrease the volume of employment and output. Thesignificance of both my present and my former argu-ments lies in their attempt to show that the volume ofemployment is determined by the estimates of effectivedemand made by the entrepreneurs, an expected in-crease of investment relatively to saving as defined inmy Treatise on Money being a criterion of an increasein effective demand. But the exposition in my Treatiseon Money is, of course, very confusing and incom-plete in the light of the further developments here setforth.

Mr. D. H. Robertson has defined to-day’s incomeas being equal to yesterday's consumption plus invest-ment, so that to-day’s saving, in his sense, is equal toyesterday’s investment plus the excess of yesterday’sconsumption over to-day’s consumption. On thisdefinition saving can exceed investment, namely, bythe excess of yesterday’s income (in my sense) overto-day’s income. Thus when Mr. Robertson says thatthere is an excess of saving over investment, he meansliterally the same thing as I mean when I say that in-come is falling, and the excess of saving in his senseis exactly equal to the decline of income in my sense.If it were true that current expectations were alwaysdetermined by yesterday’s realised results, to-day’seffective demand would be equal to yesterday’s income.Thus Mr. Robertson’s method might be regarded asan alternative attempt to mine (being, perhaps, afirst approximation to it) to make the same dis-tinction, so vital for causal analysis, that I have tried