"How far we are to consider the interests of posterity when they seem to conflict with those of now-existing human beings? The answer to this, though, seems clear: the time at which a man exists can’t affect the value of his happiness from a universal point of view; so the interests of posterity must concern a utilitarian as much as those of his contemporaries—except in that the effect of his actions on the lives and even the existence of posterity must be more uncertain."