Labor is quite a simple category. The idea of labor in that sense, as labor in general, is also very old. Yet, “labor” thus simply defined by political economy is as much a modern category, as the conditions which have given rise to this simple abstraction. The monetary system, e. g. defines wealth quite objectively, as a thing (?)[164] in money. Compared with this point of view, it was a great step forward, when the industrial or commercial system came to see the source of wealth not in the object but in the activity of persons, viz. in commercial and industrial labor. But even the latter was thus considered only in the limited sense of a money producing activity. The physiocratic system [marks still further progress] in that it considers a certain form of labor, viz. agriculture, as the source of wealth, and wealth itself not in the disguise of money, but as a product in general, as the general result of labor. But corresponding to the limitations of the activity, this product is still only a natural product. Agriculture is productive, land is the source of production _par excellence_. It was a tremendous advance on the part of Adam Smith to throw aside all limitations which mark wealth-producing activity and [to define it] as labor in general, neither industrial, nor commercial, nor agricultural, or one as much as the other. Along with the universal character of wealth-creating activity we have now the universal character of the object defined as wealth, viz. product in general, or labor in general, but as past incorporated labor. How difficult and great was the transition, is evident from the way Adam Smith himself falls back from time to time into the physiocratic system. Now, it might seem as though this amounted simply to finding an abstract expression for the simplest relation into which men have been mutually entering as producers from times of yore, no matter under what form of society. In one sense this is true. In another it is not.