Deep beneath the surface of Kant's theory of knowledge lies the metaphysical doctrine of noumena, things in themselves, intelligible entities. For lengthy periods these creatures are surprisingly unobtrusive and can be safely disregarded. But at certain points Kant hauls them to the surface and tries to put them to work in perplexing ways. My concern is not with these attempts, but with what can be learned, if not salvaged, from the metaphysical doctrine as it is expounded in the chapter in the Critique of Pure Reason entitled ‘The Ground of the Distinction of all Objects in General into Phenomena and Noumena’. I shall start by giving, for the most part in Kant's own words, as blunt an account as possible of how he appears to reduce his own doctrine to nonsense. I shall then argue that such an account, while not straightforwardly wrong, ignores matters of very great interest. I shall do so in terms of two theories, which I distinguish, about the relationship between knowing, changing and conceptualizing. Finally, I shall draw some more and less general conclusions.