All the adherents of the faculty theory maintain, however, that it is subject to certain modifications. Incomes from interest, rent, and socially occasioned increases in the value of property should be taxed at a higher rate than incomes that represent expenditures of labour; for to give up a certain per cent. of the former involves less sacrifice than to give up the same per cent. of the latter. Therefore, increments of land-value may be fairly taxed at a higher rate than salaries, personal property, or even rent and interest. When, however, the law absorbs the whole of the value increments, it seems to be something more than a tax. The essential nature of a tax is to take only a portion of the particular class of income or property upon which it is imposed. The nearest approach to the plan of taking all future increases in land value is to be found in the special assessments that are levied in many American cities. Thus, the owners of urban lots are frequently compelled to defray the entire cost of street improvements on the theory that their land is thereby and to that extent increased in value. In such cases the contribution is levied not on the basis of the faculty theory, but on that of the benefit theory; that is, the owners are required to pay in proportion to benefits received. All adherents of the faculty theory admit that the benefit theory is justifiably applied in situations of this kind. It might be argued that the latter theory can also be fairly applied to increments of land value that are to arise in the future. In both cases the owner returns to the State the equivalent of benefits which have cost him nothing. There is, however, a difference. In the former case the value increases are specifically due to expenditures made by the State, while in the latter they are indirectly brought about by the general activities of the community. We do not admit with the Single Taxers that this "social production" of value increments creates a right thereto on the part of either the community or the civil body; but even if we did we should be compelled to admit that the two situations are not exactly parallel; for the social production of increases in the value of land involves no special expenditure of labour or money. Hence it is very questionable whether the appropriation of the whole of the future value increments can be harmonised with the received conceptions and applications of the canons of taxation. _The Morality of the Proposal_