Jackson's Mary

Define Physicalism as the thesis that all the (relevant) facts are physical facts.

Fred is able to consistently sort tomatoes into two piles, because Fred is able to distinguish between two kinds of red (what he calls "red-1" and "red-2"). He calls the rest of us red-1 and red-2 color-blind, because we cannot distinguish between objects that are red-1 and objects that are red-2. Yet we can determine all the neurophysiological facts about Fred.

Mary is an expert in vision. She has, however, gained her expertise over black-and-white television while in a black-and-white room. Mary has never seen the color red, so she does not know what it is like to see red. What she does know are all the facts of neurophysiology pertaining to vision.