Discussion with Jeff Medina made me realize that I can't even buy into the model needed to ask how to time discount. That model supposes you compute expected utility out into the infinite future. That means that, for every action k, you compute the sum, over every timestep t and every possible world w, of p(w(t))U(w(t, k))_t .

If any of these things have countably many objects - possible worlds, possible actions, or timesteps - then the decision process is uncomputable. It can't terminate after finitely many steps. This is a fatal flaw with the standard approach to computing utility forward to the infinite future.