Nearly 30 years ago, Terry Horgan and Mark Timmons published a now- popular article that combines Hilary Putnam’s Twin Earth scenario with G.E. Moore’s open question argument in an effort to show that moral naturalism – the view that moral facts are at bottom ordinary, natural facts of some sort – is probably false. Responses to Horgan and Timmons’s “revised open question argument” have been legion, but surprisingly, no one has attempted to test the core assumption upon which the argument (...) is based; namely, that competent language users univocally treat natural kind-terms as rigid designators. Here, I present evidence that the intuitions of competent English speakers are not as univocal as Horgan and Timmons need them to be to ground their argument against moral naturalism. I also briefly sketch a way that the moral naturalist can respond even if competent language speakers do treat natural-kind terms in the way that Horgan and Timmons suggest. (shrink)