Incommensurability and inconsistency of languages

Incommensurable theories are said to be both incompatible and incomparable. This is paradoxical, because, being incompatible, these theories must have the same subject-matter, yet incomparability implies that their subject-matter is different. This paper's proposed resolution of the paradox makes use of the distinction between internal subject-matter and external subject-matter for languages (frameworks) as outlined by W. Sellars. Incommensurability arises when two languages share the same external subject-matter but differ in internal subject-matter. When they share the same external subject-matter, they can be inconsistent (hence incompatible), and yet incomparable (because they are about distinct internal subject-matter). A substantial part of the paper is devoted to the technical development of the notion of inconsistency as a relationship between languages in contrast to the traditional notion of inconsistency between statements.An earlier abridged version of this paper was read to the 1983 Annual Conference of New Zealand Division of the Australasian Association of Philosophy. I owe my gratitude to various people for their criticisms, especially to Andrew Holster.