They are useless without a {211} main-spring and a train of intervening wheel-work. But if any one were explaining the causes of a chronometer’s exceptional accuracy he would never think of mentioning these last at all. He would not dwell on the properties of the coil of elastic steel, or on the interaction of the ordinary toothed wheels, or on the steel axes which make their interaction possible. And why would he omit these causes? He would omit them because they would be assumed, because there would be no discussion about them, and because they are implied in the existence of all watches and chronometers equally. If, however, the case were reversed—if all escapements and all balances were alike, and there was no room for superiority except in the main-spring and the wheel-work—the latter would be dwelt on, and the former would be passed over, in any discussion that turned on the causes of accurate time-keeping. ‡ It was his complete neglect of these considerations that enabled Karl Marx to impose on himself and others his doctrine that the value of commodities depended on the amount of average labour embodied in them—a doctrine which is the most remarkable intellectual mare’s nest of the century. It is perfectly true that if all other circumstances were always equal—the demand for the commodities in question, the ability with which average labour is directed, and the assistance which the genius of the great inventors gives to it—it is perfectly true that then the amount of average labour embodied in various commodities would be the measure of their value; for labour in that case would be the only variant. But, in reality, the important variants are not average labour, but the ability by which labour is directed. The efficiency of labour itself is practically constant; and for the student of wealth-production the principal force to be studied is the ability of the few, by which the labour of the many is multiplied, and which only exerts itself under special social circumstances.