 It is difficult to formulate this point precisely, however. For instance, Oscar can think that water is wet and then think the meta-level thought, “that thought I just had was about water!”, referring thereby to H2O and thus expressing the very aspect of his original thought which distinguishes his content from Twin Oscar’s. Since neither Oscar nor Twin Oscar has thoughts about the substance his twin has thoughts about, it is not clear what it means to say that they cannot introspectively distinguish between these different thoughts. One way to try to clarify and reinforce the argument is to consider the phenomenon of “slow switching” (introduced in Burge 1988, and used by Boghossian 1989 to pose difficulties for self-knowledge). Suppose Oscar moves to Twin Earth. Initially his water-thoughts will continue to be about water, but it seems that gradually, the longer he interacts with XYZ and the longer he is out of touch with H2O, his thoughts will come to be about XYZ rather than H2O. If this is correct, then his ‘water’-thoughts will have come over time to have a different broad content than they previously had. However, this change in content will be completely invisible to Oscar himself. From his own subjective point of view, his thoughts appear to have exactly the same content as before. If there is a kind of mental content to which we have introspective access, and if introspective access must include the ability to recognize when contents are the same or different, then the sort of content to which we have introspective access cannot be broad content. This suggests that we need a concept of narrow content to capture the kind of content that we are immediately aware of.