VR: So when we say mental states are brain states, what do we mean?
Shygetz: We mean that each mental state corresponds to one and only one brain state. However, you labor under the misconception that when you think "apple" and I think "apple" we have the same brain state. We do not--when you think "apple" what color fruit are you imagining? What size, what exact shape, shiny or dull, alone or in a context? If "apple" doesn't refer to a single unique physical state, what makes you think it refers to a single unique mental one.
VR: There are, of course, various chairs, some made of different stuff and some different colors, but there is something that makes them all chairs. Doesn't everyone's thought of an apple have to have something physical in common if it is a physical state.
More, A can correspond to B without being identical to B, so there has to be more to identity than correspondence.
Causal role is determined by physical structure. If there is nothing about the property "being a thought about a pencil" that is identical to some particular physical state-type, then the mental state-type cannot be causally relevant.
Shygetz: And the only way you can do this is to posit something that interacts intimately with physical matter and can be strongly affected by physical matter while remaining somehow distinct from physical matter in some manner you have yet to even attempt to explain. I hope you are not trying to imply that dualism is more simple than physicalism, because it is not by some undefined by doubtlessly large amount. You are positing an entire new branch of physics based on a substance that violates its current laws.
VR: No problem. We need this one in order to preserve the logical foundations of science.
Shygetz: Yet again, you make a bald assertion that actually flies in the face of (admittedly incomplete) evidence. Do you have any evidence that this is true? If so, by all means present it. If not, you are not making an argument--you are declaring by fiat. Argument is necessary, sir; I don't think anyone here will be convinced by raw audacity. Show me a reason to think that physical data are insufficient to determine mental states--otherwise, you merely continue to beg the question.
VR: It's very simple really. Identity claims are necessary truths. In order for physical states to determine intentional states uniquely, it must be logically contradictory to deny the mental state once the physical information is given. Postulate any amount of physicalistic information you want, and you will never get anything that logically entails the existence of a mental state. The only way to get an argument that has a conclusion "X is about B" is to have intentional states in the premises. It doesn't matter how much physical information you give, it will always be logically possible for me to deny the existence of the mental state without logical contradiction.
The irreducibility of intentional states to physical states is held by many philosophers, many of whom, like Donald Davidson, are philosophical naturalists. There is also the argument that intentional-state attributions involve normative elements, and therefore, cannot follow necessarily from the existence of physical states. Many naturalists accept a dualism of properties but try to avoid a dualism of substances. The problem then arises as to how those nonphysical properties fit into a physical world, and also how non-physical properties can possibly be causally relevant.
Shygetz: Ah, now you are at least starting down the right path. Have you ever, and I mean EVER, added, subtracted, or manipulated powers in a mental vacuum? No; you always bring along your "unique perspective" which changes your mental state. Computers can add in a vacuum; if I take two identical computers and have one add 2 + 2, then take another computer and manipulate its physical states so they replicate the first one exactly, the second computer will have added 2 + 2. What is the reason to think that the human brain is different when adding 2 + 2?
VR: Computers have no first-person perspective. Therefore, they do not literally add 2 + 2. They do not perceive the relationship amongst the meanings. We perceive those relationships. However, physical facts are not perspectival. If my perspective determines how atoms go in my brain, we have a non-publicly accessible fact that determines physical states. That's not considered good naturalism.
It's like taking a bunch of indicative facts about the world and concluding the existence of an objectively binding moral obligation. You have to wrong type of facts on the one side to draw the proper conclusions on the other.
You have to go from facts are not subjective or perspectival, not normative, not intentional, and not purposive, and yet these facts have to entail truths that are subjective/perspectival, normative, intentional and purposive. That is a good deal more than just a question about how the bacterial flagellum got engineered.
I am the author of C. S. Lewis's Dangerous Idea: In Defense of the Argument from Reason, published by Inter-Varsity Press. I received a Ph.D in philosophy from the University of Illinois at Urbana-Champaign in 1989.