 Utilitarian distribution principles, like the other principles described here, have problems with specification and implementation. Most formulations of utilitarianism require interpersonal comparisons of utility. This means, for instance, that we must be able to compare the utility one person gains from eating an apple with that another gains from eating an apple. Furthermore, utilitarianism requires that differences in utility be measured and summed for widely disparate goods (so, for instance, the amount of utility a particular person gains from playing football is measured and compared with the amount of utility another gains from eating a gourmet meal). Some critics have argued that such interpersonal utility comparisons are impossible, even in theory, because even if all the diverse goods can be combined into a single index of ‘utility’ for an individual, there is no conceptually adequate way of calibrating such a measure among individuals (see Elster 1991).