Finally, a great economic law pointed out by Adam Smith, which has an important and almost conclusive bearing upon this vexed problem of the diffusion of taxes, should not be overlooked--namely, his statement in The Wealth of Nations that "_no tax can ever reduce for any considerable time the rate of profit in any particular trade, which must always keep its level with other trades in the neighborhood_." In other words, taxes and profits, by the operation of the laws of human nature, constantly tend to equate themselves. Man is always prompted to engage in the most profitable occupation and to make the most profitable investment. And since the emancipation from feudalism with its sumptuary laws, legal regulations of the price of labor and merchandise, and other arbitrary governmental invasions of private rights, individual judgment and self-interest have been recognized as the best tests or arbiters of the profitableness of a given investment or occupation. The average profits, therefore, of one form of investment, or of one occupation (as originally shown by Adam Smith), must for any long period equal the average profits of other investments and occupations, whether taxed or untaxed, skill, risk, and agreeableness of occupation being taken into consideration.[19] Natural laws will, accordingly, always produce an equilibrium of burden between taxed and untaxed things and persons. There is a level of profit and a level of taxation by natural laws, as there is a level of the ocean by natural laws. In fact, all proportional contributions to the State from direct competitors are diffused upon persons and things in the taxing jurisdiction by a uniformity as manifest as is the pressure upon water, which is known to be equal in every direction.