Imagine an intelligent bat contemplating the mind-body problem, name of Tim Nigel. TimNigel has noticed that humans have an auditory sense not possessed by bats (of his species): they can hear various pitches. This enables them to appreciate music (unlike Tim and his conspecifics) and also to have other types of auditory experience not available to bats. We can suppose that bats hear only a single pitch and only echoes of their own monotone shrieks, impressive though their sense of echolocation is. Thus Nigel concludes that he doesnt know what it is like to be a human, at least so far as hearing is concerned. He has some inkling, to be sure, because he does have an auditory sense, but the range and variety of human hearing makes this sense alien to himjust as humans have an auditory sense that provides only partial insight into the auditory sense of bats. He thinks thatifhe could hear pitch variations in the manner of humans,thenhe would know (fully) what it is like to be human; but as things stand he cannot grasp the nature of human experience. This is a region of reality he cannot get his mind around (Nigel is a resolute metaphysical realist). He expresses his conclusion by saying that human experience is subjective and can only be grasped from a particular point of view, in contrast to objective things that can be grasped from many points of view, i.e. from no specific point of view.
Having come to this conclusion he notices an implication for the mind-body problem, namely that experiences like those of humans cannot be reduced to physical facts about the human body and brain. For such physical facts can be grasped from many points of view and dont require that one shares the point of view of the organism having the experience. Tim can know what it is to be a human, i.e. to belong to the human biological species, but he cant grasp what it is like to belong to the psychological type exemplified by humans, i.e. beings sensitive to pitch differences. But that means that it is not possible to analyze experiences as physical states, because the former are subjective and the latter objective. He has uncovered a feature of mental concepts that renders them incapable of analysis into physical concepts. Tims inability to know what it is like to be a human thus leads him to reject materialism.
The essential point of his reasoning is the contrast between concepts of experience and concepts of the physical worldthe point, namely, that the former are accessible only to beings that share the experience in question while the latter are not dependent in this way. You can know what it is to be a member of the human species without yourself being of that species, but you cant know what it is like to have human experience without having that kind of experience. And you can grasp the properties of a human brain without yourself having that kind of brain, but you cant grasp the experiential properties with which these brain properties correlate without having those properties yourself.
That is what Tim Nigel concludes from his reflections on human experience (and which he publishes a paper on with the title of the present paperwhich quickly becomes classic of bat philosophy). I would like to rephrase the gist of his argument in a way that brings out its logic more clearly than in Nigels original formulation (not that theres anything wrong with it!). Instead of talking about subjectivity, objectivity, and points of view, I shall say that the relevant feature of (concepts of) experience isself-acquaintance dependence(SAD). The term needs some unpacking. We are familiar with the idea of concepts that depend for their possession on acquaintance with members of their extensionit is a cornerstone of empiricism. Thus it may be said that the concepts red andsquareare acquaintance-dependentyou have to experience red and square things before you can have these concepts. Putting aside the plausibility of that position, we know what it means; well, the present idea is that certain concepts require for their possession acquaintance with instances of the propertyin oneself. That is, you have to be aware of the property as instantiated by you if you are to have the corresponding concept.
So to have a concept of a certain type of experience you have to be acquainted with instances of that type in your own person: for example, you can only have the concept of an experience of red if you have yourself had experiences of red. You have to be an instance of the general property the nature of which you aspire to grasp. Such a property is what I am calling self-acquaintance dependent: you can grasp it only if you are acquainted with it in yourself. Thus for Tim Nigel the concept of pitch perception is SAD: it requires him to have a certain type of experience that he doesnt possess. He cant grasp human auditory experience because he lacks that type of experience, while he can grasp what it is to be human (i.e. that biological species). He can grasp the nature of P-fibers in the human brain but not the type of experience these fibers underlie. Thus experiences cant be brain states since the former concepts are SAD while the latter are not. The argument is by Leibnizs law: experiences have a property that brain states dont have, viz. self-acquaintance dependence of the relevant concepts.
Having got this far the astute Nigel wonders whether his argument generalizes: are all mental states SAD? He concludes that they are, since it is not possible to grasp what these states involve unless you yourself share thememotions, memories, thoughts, bodily sensations, desires, volitions, etc. It is true that, like perceptual experiences, all these phenomena have non-SAD aspects: neural correlates, functional properties, non-mental causes and effects, number and duration. But these aspects dont exhaust their nature, which always has a bit of SAD in it. For example, how could you knowfully knowwhat anger is if you had never been angry? How could a non-thinking being know what thinking is? How can memory be fully grasped if you have never remembered anything? Moreover, Nigel concludes, no other types of concept are SAD:onlymental concepts depend for their possession on instantiating them oneselfnot mathematical concepts or moral concepts or color concepts or shape concepts or aesthetic concepts. SADness is the mark of the mental (cf. intentionality or privacy or rationality).
So, at any rate, Tim Nigel contends in his famous paper What is it like to be a Human?, and his argument seems clear enough.