42 THE A B C OF TAXATION on purchase price, plus interest on any mortgage, plus taxes. Proposition 4. — Neither a tax upon ground rent, nor the ground rent itself, adds anything to the cost of land for use. (a) Economic rent, ground rent, measures the value of all public, quasi-public, and social service. If the whole ground rent is not a burden, but merely an equivalent for social values received, neither can interest and taxes, two of the parts of which ground rent in our illustration is composed, be a burden upon the user. A tax upon rent comes out of rent, which, as has been explained, is the natural tax that every user has to pay to some one, and hence it subtracts nothing from wages and adds nothing to the cost of living. Proposition 5. — You cannot pay |6,ooo for the land and in addition pay either the mortgage interest of |ioo or the tax of |ioo, because that would make land cost you I400 per annum which by our assumption is worth only $300. (a) The tax upon land cannot be added to the ground rent—which is kept at its maximum by market demand — but is a part of, and must come out of, ground rent. If it could be added, that fact would itself indicate that the ground rent was I400 instead of $300, which is contrary to supposition. Land worth only I300 a year cannot be made worth I400 a year by putting a tax of $100 upon it. (ib) Let it not be forgotten that ground rent, in the sense in which the word is used, is the same homo geneous thing, one and indivisible, the world over — what land is worth for use. It is rent —or use value — not cost of construction or cost of production —