Copyright © 1999 Massachusetts Institute of Technology
One of the most distinctive features of conscious mental states is that there is "something it is like" to have them. There is something it is like for me to smell a rose, daydream about my vacation, or stub my toe, but presumably for the rose there is nothing it is like to have an odor, and for the table leg there is nothing it is like to be kicked by my toe. Many philosophers feel that it is this feature of mentality, its subjectivity, that presents the most difficult challenge to a materialist theory of the mind.
There are two issues involved here. First, we want to know what distinguishes those states that there is something it is like to be in from those there is nothing it is like to be in; those states that have a subjective character from those that do not. Second, there is the more specific question concerning just what it is like to be in particular types of mental states. The qualitative character of smelling a rose is quite distinct from seeing it. What determines, for a state there is something it is like to be in, exactly what it is like?
Nagel (1974) poses the second question quite forcefully with the following example. Bats navigate by echolocation, a sensory system unlike any possessed by human beings. They emit high-pitched sounds and determine the spatial layout of their environment by the nature of the echoes they detect. To the question how this system performs its function of providing spatial information to the bat, a computational theory is the answer. If we can determine the function from sensory input to informational output, and see how the bat's neurophysiology implements that function, then the strictly computational question is answered. But there seems to be another question we want answered; namely, what is it like for the bat to perceive the world in this way? To this question, it does not seem as if the computational theory can provide even the beginning of an answer.
Another very influential thought experiment that illustrates the subjectivity of conscious mental states is Frank Jackson's (1982) story of Mary, the neuroscientist brought up in a black-and-white environment. Though she knows all there is to know about the physical mechanisms underlying color perception, it seems that she would not know what it is like to see red until she actually sees it for herself. Again, it seems as if one has to undergo conscious experiences to know what they are like. But why should that be?
Nagel's diagnosis of the problem is that conscious states essentially involve a "subjective point of view," whereas physicalist and computationalist theories involve adopting an objective point of view. Thus to know what it's like to have a certain conscious experience one must be capable of taking up the relevant point of view -- that is, one must be capable of having this sort of experience. The puzzle for materialism is to explain just how the physical mechanisms underlying conscious experience could give rise to subjective points of view at all, and to particular facts concerning what it's like that are accessible only from certain points of view. How objectively describable physical processes result in subjective points of view is central to the problem known as the EXPLANATORY GAP.
Two of the most influential responses to the problem are the "knowledge how" strategy and the "indexical" strategy. The idea behind the first strategy (see Lewis 1990 and Nemirow 1990) is that knowledge of what it is like to have an experience is not factual knowledge, therefore not susceptible to explanation. Rather, to know what it's like to see red is to have the ability to recognize seeing red when it occurs. If this is right, then of course one has to actually undergo the relevant experience in order to manifest the ability. Many philosophers reject this solution, however. They do not find it plausible that knowing what it's like is merely "knowledge how," an ability, and not "knowledge that," factual knowledge. It seems clear to them that there is a fact involved here, and in certain cases, such as the bat's perception via echolocation, one that is inaccessible to us.
The idea behind the second strategy is to deflate the mystery by showing that the subjectivity of conscious experience is just another manifestation of the phenomenon of indexicality (see Lycan 1996; Rey 1996; Tye 1995). Though many people can describe me -- my physical appearance, my behavior, and even many of my thoughts -- they cannot do it in a first-person way. That is, they cannot capture what I am thinking or doing by saying "I am writing an essay"; only I can do that. So, in a sense, certain ways of thinking about me are inaccessible to them. Yet, no one finds this mysterious; it is just the way the "I"-concept works. Similarly, that there is a subjective point of view is not mysterious. What accounts for subjectivity is that the subject herself has a means of representing her own experience, that is, by the mere fact that it is her experience, inaccessible to others.
This solution has its problems as well (see Levine 1997). It is true that both phenomena, knowing what it's like and employing the first-person concept, involve adopting a perspective. But it is plausible that the first-person concept is purely a matter of perspective, and that is why it generates no mystery. When one considers what it is like to see red, however, it doesn't seem that one is merely apprehending from a different perspective the same fact as that which is captured by a graph of the spectral reflectance of the object observed; rather, it seems as if something completely new is revealed, a substantive fact that is not capturable in any way by a graph or verbal description.
How seriously one treats the problem of subjectivity in the end is largely a matter of one's attitude toward philosophical intuitions. Many philosophers (see Akins 1993; Churchland 1985; Dennett 1991) are willing to dismiss the intuitions driving the worries about what it's like in the hopes that enough scientific progress will render them impotent. Other philosophers continue to engage the problem directly, convinced that the intuitions Nagel and others express reveal a deep philosophical problem that demands a solution.
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Copyright © 1999 Massachusetts Institute of Technology