 A second interesting development deals with intergenerational ethics. With an infinite horizon, it is essentially impossible to combine the Pareto criterion and anonymity (permuting the utilities of some generations does not change social welfare) in a fully satisfactory way, even when utilities are perfectly comparable. This is a similar but more basic impossibility than Arrow’s theorem. The intuition of the problem can be given with the following simple example. Consider the following sequence of utilities: (1,2,1,2,…). Permute the utility of every odd period with the next one. One then obtains (2,1,2,1,…). Then permute the utility of every even period with the next one. This yields (2,2,1,2,…). This third sequence Pareto-dominates the first one, even though it was obtained only through simple permutations. This impossibility is now better understood, and various results point to the “catching up” criterion as the most reasonable extension of sum-utilitarianism to the infinite horizon setting. This criterion, which does not rank all alternatives, applies when the finite-horizon sums of utilities, for two infinite sequences of utilities, are ranked in the same way for all finite horizons above some finite time. Interestingly, this topic has seen parallel and sometimes independent contributions by economists and philosophers (see e.g. Lauwers and Liedekerke 1997, Lauwers and Vallentyne 2003, Roemer and Suzumura 2007).