214 THE GENERAL THEORY OF EMPLOYMENT bk. IV
as the sole factor of production, operating in a givenenvironment of technique, natural resources, capitalequipment and effective demand. This partly explainswhy we have been able to take the unit of labour asthe sole physical unit which we require in our economicsystem, apart from units of money and of time.
It is true that some lengthy or roundabout processesare physically efficient. But so are some short processes.Lengthy processes are not physically efficient becausethey are long. Some, probably most, lengthy processeswould be physically very inefficient, for there are suchthings as spoiling or wasting with time . 1 With a givenlabour force there is a definite limit to the quantity oflabour embodied in roundabout processes which canbe used to advantage. Apart from other considera-tions, there must be a due proportion between theamount of labour employed in making machines andthe amount which will be employed in using them.The ultimate quantity of value will not increase in-definitely, relatively to the quantity of labour employed,as the processes adopted become more and moreroundabout, even if their physical efficiency is stillincreasing. Only if the desire to postpone consump-tion were strong enough to produce a situation in whichfull employment required a volume of investment sogreat as to involve a negative marginal efficiency ofcapital, would a process become advantageous merelybecause it was lengthy; in which event we shouldemploy physically inefficient processes, provided theywere sufficiently lengthy for the gain from postpone-ment to outweigh their inefficiency. We shouldin fact have a situation in which short processes wouldhave to be kept sufficiently scarce for their physicalefficiency to outweigh the disadvantage of the earlydelivery of their product. A correct theory, there-fore, must be reversible so as to be able to cover thecases of the marginal efficiency of capital corresponding