The relation of consciousness to the material world is puzzle, which has its origin in dualism, a philosophy of mind which posits their fundamental separation. Dualism, in turn, has its roots in folk wisdom. The belief that humans are more than bodies and that there is something in human nature that survives bodily death has its origins in prehistory; it becomes explicit in the mythology of Ancient Egypt and Assyria and was formulated into a philosophical position in the Platonic thought of Ancient Greece.
But the contemporary view that the interaction of consciousness with matter poses a problem which may be beyond scientific understanding can be traced to a clearer formulation of dualism proposed by Descartes. According to Descartes (1644) the Universe is composed of two fundamentally different substances, res cogitans, a substance which thinks, and res extensa, a substance which extends in space. Res extensa is the stuff of which the material world is made, including living bodies and brains; res cogitans is the stuff of consciousness.
Descartes maintained that, in humans, res cogitans and res extensa interact via the pineal gland, located in the center of the brain. However, even in the seventeenth century, the causal interaction of substances as different as these was thought by some to pose an insuperable problem. Leibniz (1686), for example, argued that only physical events could cause physical events and only mental events could cause mental events. Fortunately, he thought, God has arranged physical events and mental events into a pre-established harmony so that given sequences of mental and physical events unfailingly accompany each other (“parallelism”).
Consequently, there is an apparent causal interaction of mind with body rather than an actual one. This view resurfaces in the contemporary assumption that for every conscious experience there is a distinct neural “correlate. ” However, attribution of such correspondences to the workings of a munificent Deity has little appeal to the modern scientific mind. Within twentieth century philosophy and science it is far more fashionable to reduce dualism to a form of materialism, for example to assume or attempt to show that consciousness is nothing more than a state or function of the brain (physicalism or functionalism).
If either form of reduction is successful the explanatory gap left by dualism disappears, for the reason that all that needs to be explained can then be explained within the domain of natural science. Fashion, however, is beginning to change (see, for example, the debates between Dennett, Fenwick, Gray, Harnad, Humphrey, Libet, Lockwood, Marcel, Nagel, Searle, Shoemaker, Singer, Van Gulick, Velmans, and Williams in Ciba Foundation Symposium 174, 1993).
The reasons for this are many – but in essence they have to do with the realization that once one has explained everything that there is to explain about the material structure and functioning of brains, one will still be left with the problems of consciousness. To put matters crudely, one cannot find consciousness by any conceivable histological examination of the brain. Nor, as Nagel (1974) puts it, can one know what it is like to be something from a physical description alone.
In Velmans (1991a) I have considered functional explanations of consciousness, tracing functional models of the mind through from input to output and concluded that consciousness cannot be found within any information processing “box” within the brain. Consciousness accompanies or results from certain forms of processing but can be dissociated conceptually, and in most cases empirically from the processes with which it is commonly identified in the cognitive literature (perception, learning, memory, language, creativity and so on).
The same can be said of models of functioning couched in other terms, such as parallel distributed processing or the language of neurophysiology (Gray 1995; Velmans 1995a). In short, while it is likely that consciousness will eventually be found to be associated with given forms of processing, it looks increasingly likely that consciousness cannot be reduced to such processing. Or, to put matters another way, “first-person perspective facts” cannot be fully reduced to “third-person perspective facts” (cf Goldman 1993; Velmans 1991a,b, 1993a). In his recent “keynote” article (this issue), Chalmers (1995) comes to the same conclusion.
But if consciousness cannot be reduced to a state or function of the brain, how might one fill the explanatory gap left by dualism? Logically, it might be possible to reduce matter to forms of existence in the mind, for example to argue along with Berkeley (1710) that material events only exist in so far as they are perceived to exist (idealism). Idealism has its modern defenders, for example in some interpretations of the observer effect in quantum mechanics (the view that the Shrodinger wave equation only collapses into an actuality once an observation is made).
In the macroworld it may also be true that the world as-perceived only exists if there are perceivers (Velmans 1990). However, as a general theory of the ontology of macroevents this position has its own well-known problems. It might be that the material world cannot have an appearance without perceivers, but it seems counterintuitive that its very existence is similarly vulnerable. Closing one’s eyes, for example, does not seem to be enough to make unpleasant events go away.