Recent discussions of externalism about mental content have been dominated by the question whether it undermines the intuitively plausible idea that we have knowledge of the contents of our thoughts. In this article I focus on one main line of reasoning (the socalled 'slow switching argument') for the thesis that externalism and self-knowledge are incompatible. After criticizing a number of influential responses to the argument, I set out to explain why it fails. It will be claimed that the argument trades on an ambiguity, and that only by incorporating certain controversial assumptions does it stand a chance of establishing its conclusion. Finally, drawing on an analogy with Benacerraf's challenge to Platonism, I shall offer some reasons as to why the slow switching argument fails to reveal the real source of tension between externalism and privileged self-knowledge.