Frank Jackson argued, in an astronomically frequently cited paper on 'Epiphenomenal qualia '[Jackson 1982 that materialism must be mistaken. His argument is called the knowledge argument. Over the years since he published that paper, he gradually came to the conviction that the conclusion of the knowledge argument must be mistaken. Yet he long remained totally unconvinced by any of the very numerous published attempts to explain where his knowledge argument had gone astray. Eventually, Jackson did publish a diagnosis of the reasons why, he now thinks, his knowledge argument against materialism fails to prove the falsity of materialism [Jackson 2005. He argues that you can block the knowledge argument against materialism - but only if you tie yourself to a dubious doctrine called representationalism. We argue that the knowledge argument fails as a refutation of either representational or nonrepresentational materialism. It does, however, furnish both materialists and dualists with a successful argument for the existence of distinctively first-person modes of acquaintance with mental states. Jackson's argument does not refute materialism: but it does bring to the surface significant features of thought and experience, which many dualists have sensed, and most materialists have missed.