The principle of fairness holds that individuals (beneficiaries) who benefit from a cooperative scheme of others (cooperators) have an obligation to do their share in return for their benefit. The original proponent of this principle, H. L. A. Hart suggests ‘mutuality of restrictions’ as a moral basis because it is fair to mutually restrict the freedom of both beneficiaries and cooperators; so called the fairness obligation. This paper explores ‘mutuality of restrictions’, which is interpreted as a right-based and an equality-based (...) justification of the fairness obligation. It is not argued whether both ways of justifying the obligation makes a success, but that they are in need of presupposing that there is a duty for beneficiaries to do their share in return for their benefit. This suggests turning to a duty-based justification of the fairness obligation. (shrink)