Thus, if we take any commodity, we shall find that it is possible to ascertain with tolerable certainty the amount of direct labor embodied in it, but that it is equally as impossible to ascertain the amount of the indirect expenditure of labor power which entered into its making. In the case of a table, for example, it may be possible to trace with some approximation to accuracy the labor involved in felling the tree and preparing the lumber out of which the table was made; the labor directly spent in bringing the lumber to the factory, and the direct labor expended in making out of the lumber a finished table; allowance may also be made for the labor embodied in the nails, glue, stain, and other articles used in making the table. So we have a fairly accurate statement of the direct labor embodied in the table. But what of the labor used to make the tools of the men who felled the trees and prepared the lumber? What of the coal miner and the iron miner and the tool maker? And what of the numerous and incalculable expenditures of labor to make the railroads, the railway engines, and to provide these with steam-power? What, also, of the machinery in the factory, and of the factory buildings themselves, and, back of them, again, the tool makers and the providers of raw materials? It is obvious that no human intellect could ever unravel the tangled skein of human labor, and that in actual exchange there can be no calculation of the respective labor content of commodities. If the law of value holds good, it must operate mechanically, automatically. And this it does, through the incidence of bargaining and the law of supply and demand.