Frank Jackson's "Knowledge Argument" is motivated with this counterfactual: imagine Mary, the color-blind color vision specialist. Mary is an expert on all aspects of color perception: the physics of light waves, the absorption and reflectance properties of surfaces, etc., and the physiology of the eyeball, the function of the rods and cones, optic nerve, and color processing areas of the brain. In fact, let's say that Mary is the world's top expert on these matters: let's say that Mary knows the complete and correct physical description and explanation of color perception. And then remember: Mary is color-blind (perhaps her professional pursuits are a bit of overcompensation). She's never seen reds or blues. She doesn't know "what it's like" to have these phenomenal experiences. The argument purports to show that psychology cannot be naturalized. The complete and correct physical description, that includes all of the physical information, nonetheless lacks some (further) information, the knowledge of what it is like to see color. Thus if "naturalized" psychology means psychology within the bounds of physical explanation, the project founders (and Husserl was right that phenomenology is autonomous).
This argument fails to prove that psychology cannot be naturalized. The reason that it fails is that, while it is true that Mary can't say what it is like to see color, neither can anyone else. Phenomenal experience is beyond the limits of language. Ostensibly phenomenal terms ("He is in pain," "She can taste the chocolate") necessarily function on the basis of some kind of public criteria (I have been rehearsing these Wittgensteinian arguments through the last several posts. They are the "solipsism" passage in the Tractatus and the "private language" argument of the PI). In the present context note that what is at stake is what it would take to naturalize psychology, to incorporate psychology fully and seamlessly into an overall physicalist worldview. Thus the argument is essentially epistemological, not metaphysical. If it is right to say that "phenomenal description," meaning description of phenomenal experiences themselves as distinct from descriptions of things and properties in the physical world, is not possible (is conceptually incoherent), then Jackson's Knowledge Argument fails to show that psychology cannot be naturalized.