Subjectivity is a pain in the ass. Nobody knows what to make of it. You see, we have two perceptions of human beings: the perception of other human beings or ourselves as these creatures walking around the world, and the perception of ourselves from the inside--the endless procession of mental, emotional, and sensational events. Personally I don't think that there is any question that the cognitive and emotional realms emerge from the physical processes of the brain. It's not just a temporary resident. Destroy the brain and you destroy the mind. But there is something irreducible about the mind. You can look at the firing of neurons, the balances of neurochemistry, the physical brain, for as long as you want, and never know what someone is actually thinking. There was a time that we could say that you could not figure out what someone was feeling, but analysis of neurochemistry and brain scans can actually tell you a lot about this. But something about the mind remains irreducible.
I suspect that the irreduciblity is actually due to the sheer complexity of the brain. If it were possible for us to hold in our mind a picture of the sum total of all neural events in real time, we might very well be able to see how the mind arises from brain function. But we can't do this, and never will. So we're stuck with the two realms, the realm of the subjective and of ideas, and the realm of the objective, of observable objects and events. We can't draw the causal link between them because it's not one link, but billions of silmultaneous links, none of which is relevant in isolation. And because of this we get into a lot of trouble.
I heard a minister on BBC radio, for example, claim that the mind's very ability to comprehend reality is a proof of the existence of God. This is a miracle that just couldn't happen without divine intervention. Well, duh... if our ancestors had not been able to comprehend reality in any way, they would have promptly run off pursuing imaginary creatures instead of food, and that would have been the end of that. And in fact, that is precisely what some humans do to this very day. It's called superstition, and in extreme cases, psychosis. The extraordinary level of comprehension typical of mathematics and physics are extremely rare and take a lot of discipline to master, and a lot of the people who achieve this tend to be a bit deficient when it comes to noticing, say, that they're not wearing matching socks. The fact is, the majority of the human race have a fingernail grip on reality at best. So if God is responsible for making us comprehend the world, he's botched the job.
The attempt to reduce one realm to the other provides us with no end of entertainment. On the one had you have the analytics who tried to convince us that all statements pertaining to the inner world were meaningless. This made many people want to punch them, and for good reason; it's this sort of thing that gives philosophers a bad name. It takes a staggering insensitivity to insist that the private thoughts and feelings of the entire human race are meaningless. In fact, the analytic discussions were themselves phenomena of the inner world, and therefore meaningless by their own criteria. This is why Wittgenstein would sit facing the wall and chant Vedic poetry during these discussions.
On the other hand, you have the seething hordes of New Age flakes who believe that reality is only what they think it is. This is really an excuse to believe whatever you want to believe without the embarassment of evidence. It also means you can stay stupid and still be a Zen Master, a good Christian, or whatever current misinterpretation is making the rounds. An ignorance of sound reasoning makes the hermetic seal complete, and these people can stumble through life in a narcissistic haze like a bull in a china shop.
In fact, there are certain domains of knowledge that pertain to the facts of the physical world, and other domains that pertain to phenomena arising from the mental world. Beliefs are irrelevant to one but have a critical influence on the other. Believing you can fly will not make it so. You cannot move or remould matter with your mind. Opinion doesn't have much bearing in mathematics or logic either. Big Brother cannot make two plus two equal five, though many dictators have tried.
Nevertheless, belief does matter when talking about how we think and behave. It was recently discovered that Economics majors, who had been schooled to believe that people were motivated by rational self-interest, were in fact far more motivated by this than any other segment of the population. A theory of human nature may not tell us much about human nature, but it speaks volumes about the theorist. The problem with 'sciences' like social studies and economics is that they tend to produce self-fulfilling prophecies. The purpose of science is to be able to predict and control, but for social sciences prediction is control, if only enough people can be convinced that the prediction is true.
The postmodernist falacy, that everything is just a matter of opinion, is based on a kernel of truth: some opinions are presented as scientific fact when they are merely emotionally motivated beliefs. Evidence appears to support these theories because the theory leads one to consider only the evidence that supports it. Having found a few pseudo-scientific theories guilty of this, the less subtle postmodernists took this is mean that all science was pseudo-science. Magical thinking is also an exageration of something real: humans have such influence on the environment (and on the lives of others) that our beliefs, through our actions, can change the reality we live in. There is an objective reality which has nothing to do with what we want to believe, but it is also true that what we believe colours our perceptions of reality, and we tend to remake the world in accordance with our beliefs.
Due to the sheer complexity of neuralogical process that produces it, mind cannot be reduced to terms of simple physical process without the loss of essential information. So we're stuck with mind and body, not because they're separate in any way, or somehow free from each other's influence, or for any mystical or spiritual reason. We're stuck with them because we cannot talk about the world, and what it is like to live in it, without both of them.