Crusoe might possess various articles of value which cost him no labor, or cost him an entirely negligible amount, such as certain scarce fruits; but the major part of Crusoe’s wealth, let us suppose, is produced, and is freely reproducible, by his labor. The amount of labor power which he expends upon his island is variable at his pleasure within wide limits. If he choose to work only a few hours a day through the year, he will produce only a certain limited amount of the most useful things. If he add more hours per day, he will produce more of the old kinds of goods and also other different articles, luxuries as contrasted with necessities, but at any rate things of lower utility. Thus as Crusoe increases the hours of labor power put forth per day, he finds that there is a decline in the additional utility produced by each successive increment of labor. On the other hand, he finds that the pain cost or disutility of labor increases as he toils longer and longer. Crusoe will, for his average day, work until the increasing disutility of labor comes to an equality in his judgment with the decreasing utility of the things being produced. It would not be rational for him to stop at an earlier point, for then further labor would produce him a means of satisfaction greater than the “pain” of the labor itself. Nor would he labor beyond this point so that the pain would exceed the pleasure gain. Thus the utility produced by, and the disutility of, the final increment of labor in the working-day counterbalance, or we have the “marginal equivalence of utility and disutility.” The most important part of Professor Clark’s teaching is that the disutility of labor expended in producing goods is the ultimate standard of their value. To establish this thesis, it is necessary for him to show that the “effective” disutility of an increment of labor is always the actual disutility of the final increment, and that the “effective” utility produced by any increment is the actual utility produced by the final increment. If Crusoe works ten hours a day, any hour of the day’s labor will have the same effective disutility as the tenth hour. Thus if it costs one hour of labor to produce the article A, the pain cost of A is always _in effect_ the disutility of the final or tenth hour of labor, no matter in what part of the day the good A happens actually to be produced. For if Crusoe should decide to go without this article in order to avoid the hour of labor which it costs, his day would then consist of nine hours spent upon the rest of his products, and the effect would be to save himself the disutility of the tenth or last hour.[236] By shortening the working-day an hour he cannot do otherwise than save himself from suffering this marginal disutility.