  Hart (1955, 185) calls his formulation a “bare schematic outline” of the obligation one owes to other cooperators; and subsequent discussion has focused on examining what is required in order to make this outline fully accurate. As it stands, Hart’s principle invites the kinds of objections emphasized by Robert Nozick (1974, 90–95):  I am not morally required to pay for books that are thrown into my house with bills attached, or to take a day off work to entertain the neighbourhood over a public address system after my neighbours have taken it in turns to do so. Rawls’s formulation, which restricts the obligation to cases in which a benefit has been voluntarily accepted, avoids these objections. However, some writers argue that this is too tight a restriction (Arneson 1982; Cullity 1995; Trifan 2000): they maintain that when others are cooperating to produce a good that is compulsory, in the sense that once it is produced one cannot avoid receiving it (without excessive cost), one can still have an obligation of fairness to share the costs of producing it. If that is true, it keeps open the prospect (embraced by Hart but not Rawls) that political obligations can be grounded in an anti-free riding principle (Klosko 1992, 2005; Wolff 1995). However, the project of grounding political obligations in this way faces several significant obstacles. These include the objections that the operation of the state does not qualify as the kind of cooperative scheme to which a principle of fair contribution properly applies; that some citizens are sufficiently self-reliant that they do not receive a net benefit from the state; that grounding an obligation of fairness to contribute towards the cost of producing a collective good falls short of justifying the use of coercion to compel the contribution; and that an obligation to contribute to the production of a collective good does not apply when the question of which goods a group should be producing is itself contested.