The proof is as follows: All teachers of natural law, etc., have unanimously declared that the differentiation into income-receiving classes and propertyless classes can only take place when all fertile lands have been occupied. For so long as man has ample opportunity to take up unoccupied land, “no one,” says Turgot, “would think of entering the service of another;” we may add, “at least for wages, which are not apt to be higher than the earnings of an independent peasant working an unmortgaged and sufficiently large property;” while mortgaging is not possible as long as land is yet free for the working or taking, as free as air and water. Matter that is obtainable for the taking has no value that enables it to be pledged, since no one loans on things that can be had for nothing. The philosophers of natural law, then, assumed that complete occupancy of the ground must have occurred quite early, because of the natural increase of an originally small population. They were under the impression that at their time, in the eighteenth century, it had taken place many centuries previous, and they naïvely deduced the existing class aggroupment from the assumed conditions of that long-past point of time. It never entered their heads to work out their problem; and with few exceptions their error has been copied by sociologists, historians and economists. It is only quite recently that my figures were worked out, and they are truly astounding.[A] [A] Franz Oppenheimer, _Theorie der Reinen und Politischen Œkonomie_. Berlin, 1912.--_Translator._