 According to mainstream opinion the most serious problem for property dualism is the danger of being driven into epiphenomenalism. If phenomenal characters are non-physical properties and if every physical event has a physical cause and if we exclude the possibility of overdetermination (where something is caused by two different causes that are both sufficient), then, arguably, whether or not a state has a particular phenomenal character cannot have any causal relevance. But if qualia are causally impotent, how can a person know that she has an experience with a particular phenomenal character? Many take it to be obvious that a person cannot know that she now has a blue experience unless her blue experience plays a prominent causal role in the formation of her belief at issue. This particular problem has been formulated as an objection against the knowledge argument in Watkins (1989). Until some time ago Jackson was one of the very few philosophers who embraced epiphenomenalism. But Jackson changed his mind. Jackson (1995) argues that knowledge about qualia is impossible if qualia are epiphenomenal and he concludes that something must be wrong with the knowledge argument. In Jackson (2003) and Jackson (2007) he argues that the argument goes wrong in presupposing a false view about sensory experience and that it can be answered by endorsing strong representationalism: the view that to be in a phenomenal state is to represent objective properties where the properties represented as well as the representing itself can be given a physicalist account. Jackson admits that there is a specific phenomenal way of representing but he now insists that the phenomenal way of representing can be accounted for in physicalist terms. Doubts about the latter claim are developed in Alter (2007). Other possible reactions to the threat of epiphenomenalism for dualism would be either to doubt that a property dualist must embrace epiphenomenalism or to develop an account of knowledge about one’s own phenomenal states that does not imply a causal relation between qualia and phenomenal knowledge about qualia (see Chalmers 2002).