"The Social Security system may be accurately described as a form of social insurance, enacted pursuant to Congress' power to "spend money in aid of the `general welfare,'" Helvering v. Davis, supra, at 640, whereby persons gainfully employed, and those who employ them, are taxed to permit the payment of benefits to the retired and disabled, and their dependents. Plainly the expectation is that many members of the present productive work force will in turn become beneficiaries rather than supporters of the program. But each worker's benefits, though flowing from the contributions he made to the national economy while actively employed, are not dependent on the degree to which he was called upon to support the system by taxation. It is apparent that the noncontractual interest of an employee covered by the Act cannot be soundly analogized to that of the holder of an annuity, whose right to benefits is bottomed on his contractual premium payments."
