[18] In a like experience the Duke of Argyll, in his work The Unseen Foundations of Society, finds an explanation of the so-called theory of Ricardo, that the rent which a farmer of agricultural land pays as the price of its hire--that is to say, the price which he pays for the exclusive use of it--is no part of the cost of the crops he may raise upon it; a conclusion that can not be possibly true, unless it be also true that rent is paid for something that is not an indispensable condition of agricultural production. "Thus rights are in their very nature impalpable and invisible. They are not material things, but relations between many material things and the human mind and will. The right of exclusive use over land is a thing invisible and immaterial, as other rights are, and, although it is, and has been since the world began, the basis of all agricultural industry, it is a basis impalpable and invisible, whereas the material visible implements and tools, whose work depends upon it, are all visible and palpable enough, and all of which would never be were we to see them without the invisible rights upon which they depend. All of the former, in their place and order, are instruments of production; all of them catch the eye, and may easily engross the attention. On the other hand, if we are induced to forget those other elements, which are equally essential instruments of production, merely because they are out of sight, then our deception may be complete, and fallacies which become glaring when memory and attention are awakened may find in our half-vacant minds an easy and even a cordial reception." Adam Smith may be fairly considered as having fully committed himself beyond all controversy in his great work, The Wealth of Nations, to the principle that taxes, with a degree of infallibility, diffuse themselves when they are levied uniformly on the same article; and he even goes so far as to admit that a tax upon labor, if it could be uniformly levied and collected, would be diffused, and that the laborer would be the mere conduit through which the tax would pass to the public treasury. Thus he says, "While the demand for labor and the price of provisions, therefore, remain the same, a direct tax upon wages can have no other effect than to raise them somewhat higher than the tax." The German economist Bluntschli, who has carefully studied this question of the final incidence of all just and equitable taxes, is in substantial agreement with the above conclusions, but prefers to use a different term for characterizing such finality than consumption, and expresses himself as follows: "In the end taxes fall on _enjoyments_. Hence the amount of each man's enjoyments and not his income is the justest measure of taxation." (Bluntschli, vol. x, p. 146.)