‘When we have an obligation, we are required to do or omit some type of action. Sometimes we are required to do or omit this type of action for all others. Sometimes we are required to do or omit it for specified others. Sometimes we are required to do or omit the action for unspecified others, but not for all others. Obligations of the first two sorts may be thought of as having corresponding rights. Obligations of the third sort cannot plausibly be thought of as having corresponding rights. … … [W]e may have a fundamental obligation to be kind and considerate in dealing with children – to care for them – and to put ourselves out in ways that differ from those in which we must put ourselves out for adults. This obligation may bind all agents, but is not one that we owe either to all children (such an "obligation" could not be discharged) or merely to antecedently specified children. What it will take to discharge this fundamental obligation will differ with circumstances; these circumstances will in part be constituted by social and institutional arrangements that connect specific children to specific others. Fundamental obligations that are not universal (owed to all others) are, when considered in abstraction from social and institutional context, incomplete or imperfect. This is not just a matter of the indeterminacy of the act or omission enjoined by the principle of obligation, but more fundamentally of the fact that, so long as the recipients of the obligation are neither all others nor specified others, there are no right holders, and nobody can either claim or waive performance of any right. If there are any fundamental obligations that are imperfect in this sense, then there are some fundamental obligations to which no fundamental rights correspond. … … Although imperfect fundamental obligations lack corresponding rights, their fulfillment was not traditionally thought optional: the very term "imperfect obligation" tells us that. What is left optional by a fundamental imperfect obligation is selection not merely of a specific way of enacting the obligation but of those for whom the obligation is to be performed.’