How to house a mind inside a brain

The eighteenth-century French philosopher Voltaire once asked how it was that the great Newtonian heavens conform to the commands of physical law, but there remains in the universe “a little creature five feet tall, acting just as he pleases, solely according to his own caprice?” (Robinson, 1980). The question was rhetorical, of course. A century earlier, Voltaire’s compatriot René Descartes had famously offered humans an exemption from the natural order, by suggesting that causal principles applied to all intelligent behaviour in animals and all automatic behaviour in humans—such as snatching one’s hand out of a flame —but that humans alone possessed a pure thinking substance, a conscious, wilful and rational soul created by God. This soul, Descartes said, directed all voluntary movements of the body, through the so-called animal spirits. Such thinking could no longer stand, Voltaire insisted. The time had come for humans to discover and acquiesce to their place in the natural scheme of things, regardless of the outcome. Consequences aside, what would it mean, pragmatically, to put humans in their place in nature? From the beginning, the answer seemed clear: there must be no more exceptionalism. The human mind— the consciousness in each of us that peers through telescopes, scribbles calculations, falls in love, practices charity and ponders infinity—must be shown to be a product of the same impersonal forces that command the movements of the planets. This, in turn, meant that the new sciences must explain the functional relationship between human conscious experience and the small lump of living matter housed within the human skull, whose affinity for all things mental had been acknowledged even by Descartes: the human brain. More than two centuries later, Voltaire’s challenge still resonates. Today, we have high-tech brain-imaging machines, a general theory of the origin of life, a map of the human genome and a growing arsenal of pharmaceutical interventions to modulate or enhance our brain functions. However, in spite of all that, many of us still remain convinced that the truths of the brain—the logic and laws of this organ as a material entity—do not capture everything about humans. Any theory of our humanness must account for moral choice, existential passion and social contracts. It must be able to explain cathedrals, stock markets, wedding ceremonies, Shakespeare and people’s willingness to die for their God. Moreover, most of us do not yet see how to relate the mind that seems to underlie those things to the material workings of brain processes. As the philosopher Colin McGinn put it, “The mind–body problem is the problem of understanding how [a] miracle is wrought” (McGinn, 1989).