Marx saw the soul of truth in the labor-value theory, as propounded by his predecessors, especially Ricardo, and devoted himself to its development and systematization. He has been accused of plagiarizing his theory from the Ricardians, but it is surely not plagiarism when a thinker sees the germ of truth in a theory, and, separating it from the mass of confusion and error which envelops it, restates it in scientific fashion with all its necessary qualifications. This is precisely what Marx did. He developed the idea of social labor which Ricardo had propounded, disregarding entirely individual labor. He recognized the absurdity of the contention that the value of commodities is determined by the amount of labor, either individual or social, _actually embodied in them_. If two workers are producing precisely similar commodities, say coats, and one of them expends twice as much labor as the other and uses tools and methods representing twice the social labor, it is clearly foolish to suppose that the exchange-value of his coat will be twice as great as that of the other worker, regardless of the fact that their utility is equal. Labor, Marx pointed out, has two sides, the qualitative and the quantitative. The qualitative side, the difference in quality between specially skilled and simply unskilled labor, is easily recognized, though the relative value of the one compared with the other may be somewhat obscured. The secret of that obscurity lies hidden in the quantitative side of labor. Here we must enter upon an abstract inquiry, that part of the Marxian theory which is most difficult to comprehend. Yet, it is not so very difficult, after all, to understand that the years devoted to learning his trade, by a mechanical engineer, for instance, during all of which years he must be provided with the necessities of life, must be reckoned somewhere and somehow; and that when they are so reckoned, his day's labor may be found to contain, concentrated, so to speak, an amount of labor time equivalent to two or even many days' simple unskilled labor time. It may be, and in fact is, quite impossible to set forth mathematically the relation of the two, for the reason that the process of developing skilled labor is too complex to be unraveled. Of the fact, however, there can be no doubt.