the conceptual incommensurability of the rival arguments in each of the three debates. Every one of the arguments is logically valid or can be easily expanded so as to be made so; the conclusions do indeed follow from the premises. But the rival premises are such that we possess no rational way of weighing the claims of one as against another. For each premise employs some quite different normative or evaluative concept from the others, so that the claims made upon us are of quite different kinds.
