One of the cleanest and most compelling arguments against physicalism in the philosophy of mind is the “Knowledge Argument”. (Here is a quick summary. The response I am going to offer doesn’t show up there, though it fits in as a variant of the “No Learning” objection. It’s also the reply Daniel Dennett gives in Consciousness Explained.) According to this argument, thorough knowledge of the physical facts of a human being will not reveal any of the subjective states of that human being–what they feel, think, and sense. But this means there are facts about a human being that cannot be known by knowing all of the relevant physical facts. Hence, physicalism is false.

The argument is typically presented in a story about Mary. Suppose Mary is a super-smart brain scientist who is unable to see colors. She learns everything there is to know about the brain, including what the brain does when it sees colors. So Mary has all the neurophysiological facts. But then suppose she is given the ability to see colors. She sees a rose and exclaims, “Oh! So that is what red looks like!” She didn’t know what red looks like, even given her knowledge of the brain. Hence there was something Mary didn’t know about human conscious experience. Hence there’s more to it than physical facts can tell.

It’s a cute and somewhat compelling example. But it’s misleading, and the misleading part is the part in the story when it is claimed that Mary knows everything there is to know about the brain. Set aside the problem that that is quite a lot. The problem, really, is that that is not enough.

Suppose Mary is a super-smart leg-scientist. She knows everything there is to know about legs, including what legs do when they walk. One day someone asks her for the best route for walking to Las Vegas. She doesn’t know, as she has spent all her time studying leg physiology. Hence, there must be more to to walking to Las Vegas than just walking.

Well, yes, we should say, of course there is. One should consult a map of some kind, and it would be helpful to recommend sturdy walking shoes and so on. Just studying walking won’t tell you which direction to take. Similarly for studying the brain. The brain evolves and learns in a natural environment. Elements in the natural environment evolve as well, and sometimes in response to organisms’ abilities to process color information. The red stuff in the world tends to be stuff that commands attention, as red stuff is usually poisonous, or pretending to be poisonous, or yummy, or blood, or meat, or something else you should pay attention to. Part of what red is has to do with what things in the environment get perceived as red, and why. Color perception would not have evolved at all if it had not been useful for processing information.

So we should change the Mary case so that Mary knows even more. She not only knows everything there is to know about the brain, but also everything there is to know about colored objects in the environment, and what role they have played in evolution. So now Mary knows that seeing red evolved so as to alert organisms to threats and opportunities in the environment, and that seeing red things usually results in a charged experience–scary, appetizing, sexy, etc. Red is attention-commanding. As such, it had better stand out brightly against things that can usually be ignored in crisis situations – green and brown things, for example. Mary may as well learn all the ways that red and other color words have been culturally embedded as well, in poems and stories and religious ideas, so as to understand the extensive role red plays in human experience.

Now once Mary knows all that, is it as obvious that she wouldn’t know what red looks like? I’ll admit, it may seem like she still wouldn’t know exactly what it looks like. But I don’t think it is obvious that she wouldn’t know. She might well gain her color vision, see a red thing, and say, “Ah, that’s pretty much what I expected!”