Systems of Political Economy, which Represent the Produce of Land, as either the Sole or the Principal, Source of the Revenue and Wealth of Every Country". Book V: Of the Revenue of the Sovereign or Commonwealth Smith postulated four "maxims" of taxation: proportionality, transparency, convenience, and efficiency. Some economists interpret Smith's opposition to taxes on transfers of money, such as the Stamp Act, as opposition to capital gains taxes, which did not exist in the 18th century. Other economists credit Smith as one of the first to advocate a progressive tax. Smith wrote, "The necessaries of life occasion the great expense of the poor. They find it difficult to get food, and the greater part of their little revenue is spent in getting it. The luxuries and vanities of life occasion the principal expense of the rich, and a magnificent house embellishes and sets off to the best advantage all the other luxuries and vanities which they possess. A tax upon house-rents, therefore, would in general fall heaviest upon the rich; and in this sort of inequality there would not, perhaps, be anything very unreasonable. It is not very unreasonable that the rich should contribute to the public expense, not only in proportion to their revenue, but something more than in that proportion." Smith believed that an even "more proper" source of progressive taxation than property taxes was ground rent. Smith wrote that "nothing [could] be more reasonable" than a land value tax. Of the Expenses of the Sovereign or Commonwealth: Smith uses this chapter to comment on the concept of taxation and expenditure by the state. On taxation, Smith wrote, Smith advocates a tax naturally attached to the "abilities" and habits of each echelon of society. For the lower echelon, Smith recognised the intellectually erosive effect that the otherwise beneficial division of labour can have on workers, what Marx, though he mainly opposes Smith, later named "alienation"; therefore, Smith warns of the consequence of government failing to fulfill its proper role, which is to preserve against the innate tendency of human society to fall apart. Under Smith's model, government involvement in any area other than those stated above negatively impacts economic growth. This is because economic growth is determined by the needs of a free market and the entrepreneurial nature of private persons. A shortage of a product makes its price rise, and so stimulates producers to produce more