Indeed, Professor Clark nowhere explicitly asserts that productivity is an adequate rule of justice. "We might raise the question," he says, "whether a rule that gives to a man his product is in the highest sense just."[238] Scattered throughout his volume, however, are many expressions which might fairly be interpreted as answering this question in the affirmative. The statements that distribution according to product is a "natural law," and that if the labourer does not get his full specific product he is "despoiled," suggest if they do not imply that wages according to productivity is not merely the economic but the ethical norm. At any rate, the assumption of productivity as the adequate canon of wage justice, is very widely adopted, and is frequently brought forward to give sanction to insufficient rates of remuneration. Hence it has been thought well to show that the economic basis of the assumption, i.e., that the labourer gets what he produces, is unproved and unprovable. _Carver's Modified Version of Productivity_ Professor Carver makes no attempt to ascertain or state the exact physical productivity of labour as compared with that of capital, but confines his attention to what he calls the "economic" productivity of a given unit of labour in a given productive process.[239] "Find out accurately how much the community produces with his [the labourer's] help, over and above what it produces without his help, and you have an exact measure of his productivity."[240] By this rule we can determine a man's productivity not only as compared with his inactivity in relation to a given industry or establishment, but as compared with the productivity of some other man who might be substituted for him. Thus understood, productivity expresses the economic value of a man to the industrial process in which he participates. It "determines how much a man is worth, and consequently, according to our criterion of justice, how much a man ought to have as a reward for his work."[241]