Levy imposed by public authority on the incomes of persons or corporations within its jurisdiction. In nations with an advanced system of private enterprise, income taxes represent the chief source of government revenue. Income tax levied on individuals or family units is known as personal income tax. In 1799 Britain enacted a general income tax to finance the Napoleonic Wars. In the U.S. an income tax was first tried during the Civil War; the Supreme Court held it to be constitutional in 1881 but declared another income tax unconstitutional in 1894. In 1913 the 16th Amendment to the Constitution made the personal income tax permanent. The fairness of personal income taxation is based on the premise that one's income is the best single index of one's ability to contribute to the support of the government; most personal income taxes are conceived on the theory that when people's financial circumstances differ, their tax liabilities should also differ. Thus U.S. income taxes are progressive taxes, falling more heavily on those who earn more money, and individual income tax deductions are allowed for items such as interest paid on home mortgage debt, unusual medical expenses, philanthropic contributions, and state and local income and property taxes. Enforcement has been facilitated by withholding the tax from wages and salaries. See alsocapital gains tax; capital levy; corporate income tax; regressive tax; sales tax; value-added tax.