It is universally agreed that it is not completely obvious how the activity of the brain produces our sensory experiences and, more generally, how it produces consciousness. This is what Chalmers has dubbed the hard problem (Chalmers 1996). Philosophers are divided about the likely nature of the solution to this problem and whether it is, indeed, a problem at all. For a very readable account of the nature of some of their discussions and disagreements, the reader should consult the book edited by Searle (1997), with contributions by Chalmers and Dennett; the anthology edited by Shear (1997); and the collection of essays by Paul and Patricia Churchland (1998).
Our own view is that it is a plausible working assumption that some activity of the brain is all that is necessary to produce consciousness, and that this is the best line to follow unless and until there is clear, decisive evidence to the contrary (as opposed to arguments from ignorance). We suspect that our present ideas about how the brain works are likely to turn out to be inadequate; that radically new ideas may be necessary; and that well-formulated suggestions (even way-out ones) should be carefully considered. However, we also believe that while gedankenexperiments are useful devices for generating new ideas or for suggesting difficulties with existing ideas, they do not lead, in general, to trustworthy conclusions. The problem is one that should be approached scientifically, not logically. That is, any theoretical scheme should be pitted against at least one alternative theory, and real experiments should be designed to choose between them. (As an example, see our hypothesis that primates are not directly aware of the neural activity in cortical area V1, the primary visual cortex [Crick and Koch 1995].)
The important first step is to find the neural correlate of consciousness (the NCC) for at least one type of consciousness. We will not repeat here our general approach to the problem, since this has been set out in a recent update of our views (Crick and Koch 1998). In this paper we wish to venture a step further by asking what can be said about the precise nature of qualia from an introspective, first-person perspective. Another way to look at the matter is to emphasize that it is qualia which are at the root of the hard problem, and that one needs to have a clear idea of exactly under what circumstances qualia occur.
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