The ignorant observer

We propose an extension of Harsanyi's Impartial Observer Theorem based on the representation of ignorance as the set of all possible probability distributions over individuals. We obtain a characterization of the observer's preferences that, under our most restrictive conditions, is a convex combination of Harsanyi's utilitarian and Rawls' egalitarian criteria. This representation is ethically meaningful, in the sense that individuals' utilities are cardinally measurable and fully comparable. This allows us to conclude that the impartiality requirement cannot be used to decide between Rawls' and Harsanyi's positions.