It is also objected that to deprive men of the opportunity of profiting by changes in the value of their land would be an unfair discrimination against one class of proprietors. But there are good reasons for making the distinction. Except in the case of monopoly, increases in the value of goods other than land are almost always due to expenditures of labour or money upon the goods themselves. The value increases that can be specifically traced to external and social influences are intermittent, uncertain, and temporary. Houses, furniture, machinery, and every other important category of artificial goods are perishable, and decline steadily in value. Land, however, is substantially imperishable, becomes steadily scarcer relatively to the demand, and its value-increases are on the whole constant, certain, and permanent. Moreover, it is the settled policy of most enlightened governments to appropriate or to prevent all notable increases in the value of monopolistic goods, either through special taxation or through regulation of prices and charges. Taking the increment values of land is, therefore, not so discriminative as it appears at first glance.[94] Another objection is that the proposal would violate the canons of just taxation, since it would impose a specially heavy burden upon one form of property. The general doctrine of justice in taxation which is held by substantially all economists to-day, and which has been taught by Catholic moralists for centuries, is that known as the "faculty" theory.[95] Men should be taxed in proportion to their ability to pay, not in accordance with the benefits that they may be assumed to receive from the State. And it is universally recognised that the proper measure of "ability" is not a man's total possessions, productive and unproductive, but his income, his annual revenue. Now, the increment tax does seem to violate the rule of taxation according to ability, inasmuch as it would take all of one species of revenue, while all other incomes and properties pay only a certain percentage.