Full Idea
In transitions between theories words change their meanings or applicability. Though most of the signs are used before and after a revolution - force, mass, cell - the ways they attach to nature has changed. Successive theories are thus incommensurable.
Gist of Idea
In theory change, words shift their natural reference, so the theories are incommensurable
Source
Thomas S. Kuhn (Reflections on my Critics [1970], §6)
Book Reference
'Criticism and the Growth of Knowledge', ed/tr. Lakatos,I. /Musgrave,A. [CUP 1974], p.266
A Reaction
A very nice statement of the view, from the horse's mouth. A great deal of recent philosophy has been implicitly concerned with meeting Kuhn's challenge, by providing an account of reference that doesn't have such problems.