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There were a persistent few who kept picking at the problem, as Israel Rosenfield shows in The Strange, Familiar, and Forgotten: An Anatomy of Consciousness, but almost all of them came at it as critics of Descartes, eager to reduce consciousness to a physiological shadow of the brain and get rid of the last toehold of spiritual substance. Julien de La Mettrie in 1747 asserted that thought and consciousness were no evidence of spiritual substance but were only properties or functions of brain matter. The pioneer German neurologist Franz Gall linked certain kinds of thought to specific physical areas of the brain, and in 1861 Paul Broca staged a dramatic public demonstration of how damage to a particular area of the brain's left hemisphere (now known as Broca's Area) rendered certain kinds of speech impossible.
But neither Broca nor Gall attracted much interest outside of their specialties. Popular attention was riveted instead on Sigmund Freud's pursuits of the mind's pathologies, which led him and most of this century's students of the mind away from the study of consciousness and into the more dubious realms of the unconscious.
In the United States, the popular dominance of pragmatism in American philosophy also diverted interest away from study of the mind and into ways of understanding and manipulating behavior. Neither the Freudians nor the behaviorists were particularly friendly to any notions of a soul, but at least none of them spent much time trying to prove that it didn't exist.
This began to change after World War II, and one can almost pinpoint the moment when consciousness once again became a direct scientific target: the conceptualization by Alan Turing of the basic model of the computer and John von Neumann's conclusion that the computations performed by complex, integrated computers are like the functions of the brain. Hence, the brain should be understood, not as the residence of the soul, but as the hardware of a computational device. The proof of this, which became known as the Turing test, was maniacally simple: Any logical function, mathematical or otherwise, can be performed on a Turing machine; complex logical functions merely require the development of more complex Turing machines to copy them artificially; eventually, a universal Turing machine will be able to perform all the logical functions of a human being, and in such a way that an observer will not be able to distinguish between the work done by the human being and the work done by the computer. At that point, the computer will have achieved the same mind state as the human being; or, to put it another way, we will discover that human consciousness is nothing different from the high-level operations of a Turing machine.
This opened a direct route toward creating computers so sophisticated that they could beat grandmasters at chess. What was less noticeable at first was that this also opened the direct route to overthrowing Descartes' dualism and demonstrating that consciousness, instead of being the proof of spiritual substance in human beings, is only the by-product of computation—at best, the software of a mental Turing machine.
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