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One objection that could be launched against the argument made in my previous article on Plantinga’s argument against materialism is that the semantic content of a belief (e.g. the belief that snow is white) just is the neurophysiological properties associated with that belief. One could also hold the same view with experiences, e.g. that a sensation of redness just is the set of neurophysiological properties associated with that sensation. While Plantinga does have a response to this sort of view in his argument (I oversimplified his argument a bit for space), against this variety of materialism another sort of argument is appropriate.
Suppose Mary is a colorblind brain scientist who has been colorblind her whole life and she knows all the neurophysiological properties of the brain, as well as everything there is to know about the electromagnetic spectrum. A man is looking at a red light, and she monitors the man’s brain while he is having the sensation of redness, such that she knows all the neurophysiological properties involved in the man sensing redness. And yet (so it is argued) if this happened she still wouldn’t know what it’s like to perceive redness due to her colorblindness. This suggests that although certain physical states might cause someone to have a sensation of redness, it is not the case that the redness sensation just is the neurophysiology, because our hypothetical brain scientist would know all about the neurophysiology without knowing what it’s like to perceive redness.