Throughout the history of western philosophy, the "Mind-Body Problem" has been defined in many ways. Perhaps this is because the only tool that philosophy really has is the ability to re-organise thought. One definition could be: the divergence between a materialist and an idealist view of experience. We could talk immediately of "objective" and "subjective"--and later we will-- but the problem with these terms is their bias. In our materialist culture, we tend to identify objective with real. We also have a genetic investment in the real as representing survival (Freud's Reality Principle). The subjective, on the other hand, tends to be pejoratively identified with the imaginary and the merely personal, private, or interior.
The Mind-Body Problem is the confusion that arises from thinking of the subject (mind) as a subtle object (body). For instance, a memory is an "object" of consciousness, enticing us to wonder what sort of object it is, compared to the event it is a memory of. How does a mental image of a chair compare to the chair itself, and how do they interact?
Realism holds that the physical world presented to us in experience as out there, solid, and real, is what truly exists. The body, as a component of that physical reality, is the part of us that is "objectively" real. Through interactions with the nervous system, the world outside the body impinges upon it, initiating complex material reactions within the body's brain, leading to objectively observable behaviour but also to subjective experience. In the materialist view it seems that events in the world, the body and the brain must cause our "interior" experience, which is non-material while somehow a byproduct of material events. It is this sense of an interior, non-physical realm we call subjective that gives rise to the M.B.P. For, how can the physical and the non-physical interact causally? In a materialist perspective, what place is there for a non-physical realm at all? We are tempted to imagine the subjective as occupying a pseudo-material existence along side the physical, and try to ponder the interactions between them. (Isn't this how we conceive the "soul", or the "subtle bodies" of metaphysics-- as quasi-physical entities subtler than physical matter?)
In contrast, idealist world-views hold that the material world is not real, but a creation of the mind. Causality is reversed-- and with it responsibility. For it is not physical reality which causes experience "in the mind", but the mind which causes the appearance of a physical reality "out there". Mind becomes primary and causative, matter becomes secondary and illusory.
Both the materialism of science and the idealism of spiritual traditions have sought the eternal and changeless beyond the realm of experience, but have pursued diverging strategies. Science looks for unifying principles behind the diversity of sensory experience by eliminating the subject of that experience as irrelevant. Spiritual idealism holds that the one underlying principle is the subject-- consciousness, the "I", the Self. From such a perspective, the M.B.P is no longer a philosophical problem (a "category mistake"), but a rather a spiritual one-- a question of mistaken identity. We erroneously think we are the body, identifying with its perspectives and priorities. And from the body's point of view, there is an external reality which holds sovereignty over its priorities, since the body's very existence is part of that reality and contingent upon processes within it. Hence the widespread belief in our culture that mind is at the mercy of the brain, an organ of the body, which is a function of the world. Idealism reverses this sequence: mind invents the world, including the body and its brain. Perhaps philosophical maturity is simply learning to live with the incompatibility of these perspectives, passing to a subtler stance which is not committed dogmatically to either one, but which can use both as appropriate footholds.
With this preamble, consider that the realness we associate with physical objects might be a category of the mind, such as Kant held space and time to be. Consider it a quality with which mind imbues sensory experience, because mind is programmed to serve the body's need to survive and maintain itself in a zone of well-being.
The organism is in constant exchange with the rest of the physical world. Each creature receives impressions from the world which it is the job of its brain to interpret in such a fashion as to allow the creature's success in a "game" of survival. Natural selection has guaranteed that by definition only those organisms exist which play this game successfully.
Now, a game is a structured activity within a playing space or field, with playing pieces and rules. Chess and Monopoly are examples. So are algebra and geometry, business and economics, computer software and hardware, human relationships, and just about anything you can think of that involves ordered, structured, describable activity. In the game of survival (or natural selection), the playing piece is the organism and the field is the environment. The rules are numerous (perhaps infinite), including the laws of physics, chemistry, and genetics. And there are many levels of rules, including what might be called the logic of the organism. To use a computer metaphor, how is it programmed? The algorithms by which it operates are the rules governing its behaviour, which are its strategies in the game of survival. In a computer game, some software is embodied in the circuitry of the hardware. So with the brain/body, its "logic" is partially hard wired in the neurology and chemistry of the organism. Whereas a human programmer created the design of the machine, eons of cumulative genetic experience (trial and error) created the capacity of the organism to play the game of life.
The brain is an organ of survival, and some creatures rely more heavily on it than others. The big-brained creatures tend to be visually and acoustically well-developed. What these two senses have in common is space. They are about spatial orientation and therefore movement-- the movement of pursuit and evasion, the hunter and the hunted. The concept of space also implies the independent existence of distinct objects-- and therefore ideas of objectivity and the material reality of the world.
Of course, we take for granted that the world simply is as our big brains depict. We believe that the space we perceive is real, that the objects in it are real, and so forth, without questioning what "real" means-- or what "meaning" means, for that matter. But perhaps the automatic 3-D realness of perception is itself a survival strategy, like physical pain. Have you ever wondered why pain "hurts"? The hurting is the way we experience the mind's judgment (and alarm!) that the body's tissues are sustaining damage. When you think about it, our physical existence cannot be free from pain, because it is the alert of pain that has allowed us to survive. And the realness with which the mind imbues physical reality is the way we subjectively experience the mind's judgment that the playing field of the world is a very serious place for the body. It is an expression of our belief that the world holds over us the power of life and death, pleasure and pain. It expresses our earnest engagement in the game of survival. Like pain, the sense of reality is functional. We are here to think casually about all this only because we take the Reality Principle (as Freud called it) seriously enough to have survived.