Questions about intentionality can be divided into two groups. In logically first place there are questions about what it is, how it is possible, and what ontological resources are required to render it intelligible. And then there are more specific questions about what implications intentionality has for the mind-body problem. Does it, for example, rule out materialism? In What is it Like to be Human (Instead of a Bat)? Laurence BonJour mounts an argument from intentionality against materialism. I will quote just the bare bones of his argument, leaving aside many of the supporting considerations:
Suppose then that on a particular occasion I am thinking about a
certain species of animal, say dogs -- not some specific dog, just
dogs in general (but I mean domestic dogs, specifically, not dogs
in the generic sense that includes wolves and coyotes). The Martian
scientist is present and has his usual complete knowledge of my
neurophysiological state. Can he tell on that basis alone what I am
thinking about? Can he tell that I am thinking about dogs rather
than about cats or radishes or typewriters or free will or nothing
at all? It is surely far from obvious how he might do this. My
suggestion is that he cannot, that no knowledge of the complexities
of my neurophysiological state will enable him to pick out that
specific content in the logically tight way required, and hence
that physicalism is once again clearly shown to be false.
[. . .]
Suppose then, as seems undeniable, that when I am thinking about
dogs, my state of mind has a definite internal or intrinsic albeit
somewhat indeterminate content, perhaps roughly the idea of a
medium-sized hairy animal of a distinctive shape, behaving in
characteristic ways. Is there any plausible way in which, contrary
to my earlier suggestion, the Martian scientist might come to know
this content on the basis of his neurophysiological knowledge of
me? As with the earlier instance of the argument, we may set aside
issues that are here irrelevant (though they may well have an
independent significance of their own) by supposing that the
Martian scientist has an independent grasp of a conception of dogs
that is essentially the same as mine, so that he is able to
formulate to himself, as one possibility among many, that I am
thinking about dogs, thus conceived. We may also suppose that he
has isolated the particular neurophysiological state that either is
or is correlated with my thought about dogs. Is there any way that
he can get further than this?
The problem is essentially the same as before. The Martian will
know a lot of structural facts about the state in question,
together with causal and structural facts about its relations to
other such states. But it is clear that the various ingredients of
my conception of dogs (such as the ideas of hairiness, of barking,
and so on) will not be explicitly present in the neurophysiological
account, and extremely implausible to think that they will be
definable on the basis of neurophysiological concepts. Thus, it
would seem, there is no way that the neurophysiological account can
logically compel the conclusion that I am thinking about dogs to
the exclusion of other alternatives.
[. . .]
Thus the idea that the Martian scientist would be able to determine
the intrinsic or internal contents of my thought on the basis of
the structural relations between my neurophysiological states is
extremely implausible, and I can think of no other approach to this
issue that does any better. The indicated conclusion, once again,
is that the physical account leaves out a fundamental aspect of our
mental lives, and hence that physicalism is false.
I will now sum up BonJour's reasoning in my own way.
BonJour is thinking about dogs. He needn't be thinking about any particular dog; he might just be thinking about getting a dog, which of course does not entail that there is some particular dog, Kramer say, that he is thinking about getting. Indeed, one can think about getting a dog that is distinct from every dog presently in existence! How? By thinking about having a dog breeder do his thing. If a woman tells her husband that she wants a baby, more likely than not, she is not telling him that she wants to kidnap or adopt some existing baby, but that she wants the two of them it engage in the sorts of conjugal activities that can be expected to cause a baby to exist.
BonJour's thinking has intentional content. It exhibits that aboutness or of-ness that recent posts have been hammering away at. The question is whether the Martian scientist can determine what that content is by monitoring BonJour's neural states during the period of time he is thinking about dogs. The content before BonJour's mind has various subcontents: hairy critter, mammal, barking animal, man's best friend . . . . But none of this content will be discernible to the neuroscientist on the basis of complete knowledge of the neural states, their relations to each other and to sensory input and behavioral output. Therefore, there is more to the mind than what can be known by even a completed neuroscience. Physicalism (materialism) is false.
I of course agree.