Some philosophers argue it is logically possible-- that is, conceivable-- that the various colors experienced by a person with normal color vision could be systematically interchanged in the experience of someone else, in a behaviourally undetectable way. This is like saying that the words of the English language could have systematically different meanings for different people, with no one the wiser. This is no doubt true in practice-- within very narrow limits-- because of the casual way in which language is used. But if discrepancies are large enough they are bound to be noticed under close enough scrutiny. I cannot expect to point to the leaves of a tree in springtime and call them red. Of course, the argument is usually made that one could have all the same names for objects in the world and still experience them differently. Thus, I could see a color that you experience as green and call green (springtime leaves), and I could call it green myself, yet experience it as what you experience as red (ripe tomatoes). This presupposes the idea of "me" somehow experiencing "your" experience. Apart from the possibility of my brain being connected to your eyes, I do not see any meaning possible for this idea. That it appears meaningful to some probably arises from their mistaking experience as a thing or domain existing objectively and independently of subjects-- a free-floating mental substance to which various observers could have access, as they do to the physical world.
Pain is often considered paradigmatically subjective and private. I cannot directly experience your pain-- obviously because my brain is not hooked up to your body. But it is not information about your tissue damage that is missing from my perception, for I could invent an instrument to extend, if necessary, my knowledge of your condition. The deeper issue is that I am not motivated to respond to your condition in the way I would be if it were my own. It is no question of privacy, no inalienable realm of one's "own" experience as privileged knowledge. Your tissues and mine are alike public domain. It is rather the relationship which differs, because this brain was designed in its responses to be concerned for this body. It is a question of values (intentions) rather than knowledge. Suppose my brain could be hooked up to your body. In that case it seems certain I would feel "your" pain as my own. Both our brains could be hooked up to both our bodies, allowing a simultaneous experience of the condition of each. Whose pain is it, then? Indeed, whose body?
Supposedly one experiences only one's own unique and private experience. But from a commonsense perspective, we all experience the same world, from differing vantages in space-time and in our own way. If all cognitive systems were qualitatively identical, and only numerically different, then all would perceive the world in the same way, differing only in spatial-temporal perspective. There could be disagreements about the world (whether, for example, it was night or day) but not different ways of experiencing it. Owing to a unique space-time locus, I could be experiencing pain while you are not. My foot could have been run over by a passing vehicle while you, standing a few feet away, were untouched. This does not make pain a private experience. I could be seeing the blue of the sky while you are seeing the dark of night. This does not make vision a private experience. My pain is an awareness of something physical and quite public--namely, damage to this body. Damage to that body will not be experienced by me as pain, but it will be experienced in some other way if I am nearby. The sky will not be experienced by me as blue if I am on the dark hemisphere of the earth. Of course, if the nerves connecting your eyes to your brain could be magically extended halfway around the earth, so that your eyes were right beside mine even though our brains were separated by thousands of miles, then you and I could see the same sky (though not precisely at the same time, owing to the finite speed of nerve impulses). Similarly, if nerves leading to your pain centers were connected to receptors in my wounded foot, you would experience the same pain as I. What then could it mean to say that we are each having a private experience of pain? What does it mean to say we are each having a private experience of blue? Is it really blue we are experiencing, as something in a private show, you in your tent and I in mine? Or is it the sky we are seeing, the common objective sky? Are we having separate but equal pains, or is it the objective damage to this foot we both feel (and see as well)? What makes experience personal is that it is for this organism. This is because the connections-- the pathways of information processing, both intentional and causal-- are within this organism. One could say, by the same token, that my knowledge of physics is for my personal use and information, as is yours for you. Should we then conclude that there is no objective science of physics or no world it is about?
In regard to both the inverted spectrum argument and the supposed privacy of pain, we could also consider scenarios where my brain is hooked up to inclusively more of the afferent pathways in your nervous system than just the pain receptors or the retina. Ultimately, what motivates this kind of thought play is wondering what my experience could be if my brain was hooked up to "all" of your brain? But what could that look like from the outside (let alone the inside)? What could it possibly mean for two brains to be "hooked up together"? Where would the joining nerves run exactly, given that brains are thoroughly decentralized?
The real difficulty posed by the apparent privateness of experience is not that it is a domain of privileged information, owing to hard connections between brain and body. Information is rather a public domain. What actually yields the sense of inviolable privacy is the possibility of deliberate misrepresentation that inheres in subjectivity, as well as in the ambiguities of language, so that the other is kept guessing as to one's inner state. Deception, bluffing, managing the other's image of oneself, are subjective stratagems; but even these are no ultimate weapons of privacy. Knowledge of other minds, like other types of knowledge, is a question of adequate models and of sensitivity to the information available. In poker, the cards are hidden, but not the faces and actions of the other players.
The inside and outside perspectives are different modes of collecting the same information. The neurophysiologist's determination that my nerves and brain respond in certain patterns to light of a particular wavelength is the same information as my determination that I am seeing blue. The intimate's "divining in a smile the oil of tears" is a perception of behaviour indicating the sadness another feels, and perhaps denies feeling. But a further difficulty arises because of the mind's imprisonment within its own systems of meaning. It is hard, for instance, to assess the words and actions of other people with whom we disagree because we can never be certain to what extent the situation is really as we see and feel it to be, or to what extent we are coloring it with our own biases or filtering it through our own defences. Each person may tell an entirely different story based on what are presumably the same objective events. As subjective beings we know that the mind is "prejudiced", to use Descartes' old fashioned term-- a system operating on particular assumptions. Other subjective minds have other thoughts, for which they claim truth as vigorously as we. And yet we know (as presumably they know) that another viewpoint is always possible, and that by invalidating the other's perspective, each could be missing something vital to their own perception of the truth. In this very personal sense, the problem of subjectivity is to distinguish what is of oneself from what is of the world-- a project begun in early childhood, and never really complete. It is the task of sorting out what we are imposing on (or excluding from) experience from what is imposed on it by others and by reality. The MBP is expressed in every act of brutality, in every gesture treating the subject as object, in every refusal to consider the experience of the other.