Scepticism about self-knowledge redux

Abstract

I would like to take another stab at defending the coherence of scepticism about self-knowledge. A possible world in which I inhabit Twin Earth is, in Gary Ebbs's phrase, a subjectively equivalent world for me, in the sense that if I were on Twin Earth, I would receive the same sensory stimulation that I actually receive, though the Twin Earth environment is different from what I actually take my environment to be.1 Accordingly, on Twin Earth, my sincere utterances of the sentence ‘There is water in the basement’ express the thought that there is twin-water in the basement, rather than the thought that there is water in the basement. One might maintain that without empirical investigation of my environment, I do not know that I am not in a world in which I inhabit Twin Earth (or in some other subjectively equivalent world). Thus, in the absence of empirical investigation: