The knowledge argument usually takes the form of a thought experiment where the subject, having some psychological deficiency, lacks any introspective data to derive the knowledge of her experience. Most defenders of the knowledge argument see the argument as both a support of dualism and an objection to any full-blooded form of physicalism. However, this paper argues that the knowledge argument against physicalism may be directed, in an exactly parallel form, against reductive dualism; moreover, although most physicalists who are the opponents of the knowledge argument do not give any convincing response to the knowledge argument, some kinds of physicalism can live with the knowledge argument.