Enter one of the more trafficked arguments against physicalism. The “Knowledge Argument” as it is sometimes called is deceptively simple and carries a fair deal of intuitive weight. It claims to show that a person can learn all the physical facts about the universe and still not know everything there is to know. In other words, there are facts about the universe that are not reducible to physical bits of knowledge. Here is a version of the argument along with a friend’s explanation of the thought experiment behind it. (I take full responsibility for any and all misrepresentations and so forth...)
We have poor Mary, an uber-brilliant neuroscientist who, from birth on, is confined to a black and white room in which she receives instruction and tutoring (by TV, radio, books, etc. - all in black and white). These lessons teach her all about physics - that is, she comes eventually to learn everything there is to know about physical facts. Or to put it differently, at a certain point in time, Mary knows all physical facts.
Now physicalism is the (ontological) thesis that everything that exists is physical. Or to put it differently, physicalism says that all facts are physical facts. Given the thought-experimental setup, we've stipulated that Mary has learn all the physical facts (according to an "ideally completed" physics, some years in the future). A fortiori, Mary knows all the facts.
Once she's done learning all of (an ideally completed) physics, Mary is released from the room (free at last!) Soon thereafter she encounters a bright red flower (or tomato, etc.). Mary exclaims "WOW! So THAT'S what it's like to see red!" It seems Mary has learned something new. But if Mary knew all the physical facts already, and if physicalism states that all facts are physical facts, then Mary coming to learn an additional fact - what the experience of red looks like - implies physicalism is false. Some facts are not physical facts (e.g. this new "phenomenal" fact Mary comes to know), so physicalism is false.
[Assumption: physicalism says all facts are physical facts; assume the facts stipulated in the Mary scenario]
Premise 1: Before leaving the room, Mary knows all the physical facts
Premise 2: After leaving the room, Mary learns a new fact
Conclusion: Therefore, not all facts are physical facts; Therefore, physicalism is false.
As I mentioned earlier, this argument carries a lot of intuitive weight (which typically counts for something in the philosophical world), but there are a few counter arguments. I will list those in my next post on this subject, but at this point I want to raise some other thoughts.
I have liked the thrust of this argument since I first encountered it. It seems to me that there is an important difference between knowing the wavelength of the color red-being able to describe it mathematically-and being able to visually pick out a bright red flower. When we use the words “bright” and “red” for instance, do we have a wavelength equation in mind, or do we have an experience in mind? Physically both “bright” and “red” can be quantified scientifically, but the experience cannot be captured in the equations.
Another concept that might be helpful at this point is the “Identity of Indiscernibles.” This is a deceptively powerful axiom which states that if two described things have all the same properties, then they are not two, but one. (The common response at this point is, “No duh.”) The present import of this idea comes to bear when we ask the question, "do the mathematical equations of 'bright red' have all and only the same properties as the experience of 'bright red'?" According to what we have seen thus far, they do not, and therefore, they are not the same thing.