Controversy continues to attach to the question whether an externalism about mental content is compatible with a traditional doctrine of privileged self knowledge. By an externalism about mental content, I mean the view that what concepts our thoughts involve may depend not only on facts that are internal to us, but on facts about our environment. It is worth emphasizing, if only because it is still occasionally misperceived, that this thesis is supposed to apply at the level of sense and not merely at that of reference: what concepts we think in terms of -- and not just what they happen to pick out -- is said by the externalist to depend upon environmental facts. By a traditional doctrine of privileged self knowledge, I mean the view that we are able to know, without the benefit of empirical investigation, what our thoughts are in our own case. Suppose I entertain a thought that I would express with the sentence `Water is wet'. According to the traditional doctrine, I can know without empirical investigation (a) that I am entertaining a thought; (b) that it has a particular conceptual content, and (c) that its content is that water is wet .