This left him with the great interest difficulty with which he occupied himself in his first chapter.[255] It remained for J. B. Clark to point out that the marginal product of labor could be disentangled from the product of capital as well as from that of land.[256] Upon this possibility depends the important productivity theory of wages. In order to explain the process by which the pure marginal product of labor is found by the entrepreneur, Clark adopts what is virtually the business man’s conception of capital, as distinguished from concrete capital goods. The latter alone have been designated capital by most economists in their formal and explicit definitions. Professor Clark prefers to call the two concepts simply capital[257] and capital goods. Capital is a “sum of productive wealth, invested in material things which are perpetually shifting—which come and go continually—although the fund abides.”[258] These material things are the capital goods. Capital as an amount must be measured by its exchange value. A capital of $100,000 may be prepared to employ say 40 men. Should it be rearranged to employ 20 men, its concrete make-up would have to be altered. A less number of machines and tools of better quality would have to compose it. Now as the concrete tissue of a given capital perishes or matures and frees its value for reinvestment in more concrete goods, an entrepreneur has it open to him to alter the concrete constitution of his capital. In this way, in the course of time, an entrepreneur may be able to rearrange his capital so as to augment or decrease the labor force employed with it. In many cases pretty large changes in the labor supply employed with a given capital could be made with little or no alteration of its technical concrete make-up. Somewhat slowly and under this and that frictional difficulty, the experimentation is made which reveals the marginal product of labor. The process which discloses this must always in the end be one in which an increment of labor is added to or removed from the force working with a given capital and an observation made of the resulting addition to or subtraction from the total product. The exposition of this process and the explanation why competition tends to make the wages of labor (of whatever grade) equal to its specific or marginal product, is probably the greatest contribution to economics contained in Clark’s _Distribution to Wealth_, and occupies a large part of that work.