A little more on subjectivity
I have always felt intuitively that the idea of the first-person private sphere is overly stated. There may be a significant impracticality to understanding what is happening in the brain-that-is-you. That is, there may be an impracticality to understanding your experience. But assuming that we as individuals are machines programmed by our histories, with our own models and representations, and endless personal knowledge and therefore associational structures that are only affiliated to us as individuals, then the idea that you have some impossible-to-see first-person experience makes sense.
Except that I strongly deny that it makes sense, or at least, that it gives us any epistemological purchase about the nature of consciousness. The reason why I am walled off from your experience is because in order to have “your experience” I would have to be a machine built by genes and environment to be the exact machine that you are at a given moment within that exact environmental space. Through story telling and background sharing and explaining the connections of the immediate environment that we as individuals are fixated on, one individual can share certain relational aspects of their experience with any other. In other words, there is a reason why us Americans like the same television shows. They elicit some similar experiences that we all share. Enough of our modeling, association structures, and perceptual structuring is shared, and therefore great similarities in our current representation of the present moment, say a TV show, is shared.
Through nature and culture similar machines (us people) are built across a society or a family. Therefore our experiences are close enough aligned to elicit similar reactions. That seems like a banal statement except so many people hang so much on what such subjectivity amounts to.
Now, can I experience the show in the exact same way as the person sitting next to me experiences it? No. But again, given the kind of machines that we are that are delicately programmed over many years to form the exact repertoire of associations and emotions that we do when we are present in a given environment, to enter into your exact experience would require me to have your (near) exact programming. What walls me off from understanding in perfect detail “your experience” is not some divide of the specialness of consciousness. It is instead the practical impossibility of programming enough of your associations and models to elicit the exact way that your perceptual representations and brain/body processes will play out.
Given the internal model or representation of what it is like when two humans experience a needle in the arm, including shared structures of pain response as well as shared cultural inculcation of pain response behavior (etc.), our internal representation, our experience, will have certain shared qualities. It will have other personal, subjective qualities that other people do not have because their associational, representational, and bodily responses are slightly different. For example, two ten year old's who are scared of shots will share more inner representational similarities than those ten year old's will share with the greatest stoic out there while she is getting a shot. Though, still, there are surely at least some inner representational similarities, for instance, some aspects of what-it-is-like when someone touches you on the arm will be shared by all three. We can assume this is partially true given that we are all generally wired in similar ways, our bodily representation models share similarities, say.
The simple story is that experience, that what it is like, is some kind of internal monitoring of our conglomerate representations. These ten years old's have significant representations that are centrally focused on the representations of, “touching arm; this is supposed to really hurt.” And the stoic has significant representations that internally get focused on, “touching arm, pain means nothing in the world.” I put the “representation” in linguistic form there, but obviously there is immense complexities there which only at times are influenced by social, linguistic mediation.
The individuality and uniqueness of our experience, of our internal representations of our self as we interact with the world is the only way that possibly makes sense. If my computer was internally representing everything within its self and what processes it was carrying out, it would quickly be having unique representational structures. It is the only computer with its repertoire of documents and programs.
Likewise, if a rock was given a modeling and representational system, it mapped the information of different parts of its self, it would be the only one with an internal representation of atoms in that exact arrangement. If we then started talking about the rocks awareness of its own self, it would be the only one that was presently mapping that exact arrangement, as well as internally representing that it is mapping that mapping of that exact arrangement. Hence, it is subjective. It also makes perfect sense that its internal representational system (its information system) may have significantly different qualities than the next rock over. Say one rock is using a scanning electron microscope to array its atoms location and it draws such maps in crayons which it then internally is shown a partial, singularly focused map of (its center of awareness). The other rock uses an internal fMRI and draws maps or representations in computer models which it then accesses and is centrally focused on (or represents its attentional system as singularly focusing on). The individuality of those internal representations, and the specificity of the given awareness beam, is going to follow from those individual structural systems.
From these examples, it follows that the internal representation of the bat is likely more walled off from visual creatures than the internal representation of visual creatures is to other visual creatures. We may be able to glean some similar inner representational similarities, say if bats have spatial mapping that is something similar to our spatial mapping, there may be the slightest overlap in what our representational systems are doing. In those ways perhaps a bat and a human will have some similar experiences, like the kids and stoic getting a shot above. That there may also be unimaginable differences in internal representation because of significant structural features, in this case perceptual differences, is no more a useful insight than that you can never fully internally represent the exact experience of the person next to you.
Lastly, language may be needed for some of the most acute internal representations and modelings. A bat and dog may have awareness and have robust experiences, but only humans (that we imagine for now) have an awareness, a representation, that we have awareness. There is good reason to believe that language provides much of the scaffolding for that kind of self-awareness. Language allows for an explosion of representing our selves as we are representing, and for delegating and dictating in finer detail all of the distinctions in the world, including as we attend to those different distinctions. It allows for thoughts on thoughts. For us linguistic beings, we may have a far tougher time imagining the experience of any nonlinguistic being than we do trying to understand specifically the bats perceptual differences.