The causal theory of reference is often taken to provide a solution to the problems, such as incomparability and referential discontinuity, that the meaning-change thesis raised. I show that Kuhn successfully questioned the causal theory and Putnam's idea that reference is determined via the sameness relation of essences that holds between a sample and other members of a kind in all possible worlds. Putnam's single ‘essential' properties may be necessary but not sufficient to determine membership in a kind category. Kuhn (...) argued that extension is fixed by similarity-dissimilarity relations that are liable to change in taxonomic reorganizations of science. *Received October 2009; revised January 2010. †To contact the author, please write to: Institute of Philosophy, University of Leiden, Martin de Vrieshof 4, 2300 RS Leiden, The Netherlands; e-mail: [email protected]. (shrink)