The principle that any tax should fall on those who can afford to pay. In practice, taking account of ability to pay means that tax payments increase with the observed income or assets of taxpayers. In principle, earning capacity should also be taken into account. The main objections to the ability to pay criterion are that earning capacity is unobservable, and that taxing income reduces the incentive to work. However, the collection of taxes from those who cannot afford to pay is unpopular, expensive, and sometimes impossible. Ability to pay is an alternative to the benefit principle under which only those who benefit from any given public expenditure should be taxed to pay for it. The government requires tax revenue to pay for public goods and income redistribution. Given the level of revenue necessary to run a modern society, use of the ability to pay criterion for taxation seems inevitable.