Equal treatment for the present and the future was required in two axioms introduced in Chichilnisky [12] [15]. We provide a characterization of the decision criterion that satisfy the axioms and show that the two axioms are equivalent to physical limits in the long run future. We prove that maximizing discounted utility with a long run survival constraint is equivalent to maximizing a criterion that treats equally the present and the future. The equal treatment axioms are therefore the essence of sustainable development. The "weight" Lambda given to the long run future is here identi.ed with the marginal utility of the environmental asset along a path that narrowly misses extinction. An existence theorem is also provided for optimizing according to the welfare criterion that treats equally the present and the future. We show that no prior welfare criteria satisfy the axioms for sustainable development introduced in [12].