A philosophical zombie is functionally identical to something that has a mind. It reacts in the same way, but it is strictly physical and does not have qualia, units of conscious experience. Jackson's knowledge argument against physicalism is that physicalists' claim that "all facts are physical facts" is false because there is a subjective character of experience, therefore there is a difference between "knowing what red looks like" and "how red looks". Raffman's criticism of Jackson's argument is that they mean the same and what is different is "what it's like to see red", the qualia, the specific example of experience. In Jackson's thought experiment, Mary gains new knowledge by learning "what it's like to see red", an experience that was