Marx

"If the mere quantity of labor functions as a measure of value regardless of quality, it presupposes that simple labor has become the pivot of industry. It presupposes that labor has been equalized by the subordination of man to the machine or by the extreme division of labor; that men are effaced by their labor; that the pendulum of the clock has become as accurate a measure of the relative activity of two workers as it is of the speed of two locomotives. Therefore, we should not say that one man’s hour is worth another man’s hour, but rather that one man during an hour is worth just as much as another man during an hour. Time is everything, man is nothing; he is, at the most, time’s carcase. Quality no longer matters. Quantity alone decides everything; hour for hour, day for day; but this equalizing of labor is not by any means the work of M. Proudhon’s eternal justice; it is purely and simply a fact of modern industry.

In the automatic workshop, one worker’s labor is scarely distinguishable in any way from another worker’s labor: workers can only be distinguished one from another by the length of time they take for their work. Nevertheless, this quantitative difference becomes, from a certain point of view, qualitative, in that the time they take for their work depends partly on purely material causes, such as physical constitution, age and sex; partly on purely negative moral causes, such as patience, imperturbability, diligence. In short, if there is a difference of quality in the labor of different workers, it is at most a quality of the last kind, which is far from being a distinctive speciality. This is what the state of affairs in modern industry amounts to in the last analysis. It is upon this equality, already realized in automatic labor, that M. Proudhon wields his smoothing-plane of “equalization,” which he means to establish universally in “time to come!”

All the “equalitarian” consequences which M. Proudhon deduces from Ricardo's doctrine are based on a fundamental error. He confounds the value of commodities measured by the quantity of labor embodied in them with the value of commodities measured by “the value of labor.” If these two ways of measuring the value of commodities were equivalent, it could be said indifferently that the relative value of any commodity is measured by the quantity of labor embodied in it; or that it is measured by the quantity of labor it can buy; or again that it is measured by the quantity of labor which can acquire it. But this is far from being so. The value of labor can no more serve as a measure of value than the value of any other commodity". 

Karl Marx