“... the notion that happiness and suffering are morally symmetric deserves our most meticulous scrutiny. It may, of course, seem intuitive to assume that some kind of symmetry must obtain, and to superimpose a certain interval of the real numbers onto the range of happiness and suffering we can experience — from minus ten to plus ten, say. Yet we have to be extremely cautious about such naively intuitive moves of conceptualization. … [I]t is especially true when our ethical priorities hinge on these conceptual models; when they can determine, for instance, whether we find it acceptable to allow astronomical amounts of suffering to occur in order to create “even greater” amounts of happiness.”