In economic analysis, it is usually assumed that each individuals well-being (mental welfare) depends on her or his own resources (material welfare). A typology is provided of the ways in which one persons well-being may depend on the material resources of other persons. When such dependencies are taken into account, standard Paretian analysis of welfare needs to be modified. Pareto efficiency on the level of material resources need not coincide with Pareto efficiency on the level of well-being. A change in (...) economic conditions that is Pareto efficient in the standard sense, i.e., with respect to material resources, may nevertheless sacrifice one persons well-being to that of another. It is shown that under plausible assumptions, Pareto efficiency on the level of well-being may require the reduction of inequality on the level of material resources. (shrink)