When you fall into deep sleep, what happens to your consciousness, to "you"? Does it hibernate or dissipate or what? Does it still exist? Your body continues to exist, your brain continues to control your body's rhythms - otherwise you would never wake up - but do "you" exist in oblivion?

The celebrated physicist Richard Feynman became interested in this question while he was a student at the Massachusetts Institute of Technology in the 1930s. He asked himself, do my thoughts suddenly stop as I fall asleep, or do they move less and less rapidly, or what? Feynman decided to study himself falling asleep. He observed that "the ideas continue, but they become less and less logically interconnected. You don't notice that they're not logically connected until you ask yourself 'What made me think of that?' and you try to work your way back, and often you can't remember what the hell did make you think of that!" After four weeks of self-experiment, Feynman concluded that, while it was possible to watch himself falling asleep, "I don't really know what it's like to fall asleep when I'm not watching myself". He composed a short verse on the problem of introspection: I wonder why. I wonder why.

I wonder why I wonder.

I wonder why I wonder why

I wonder why I wonder!

How can "I" observe "I"? What are we really seeing when we stare into the depths of our own eyes in a mirror? Another great physicist, Erwin Schrodinger, one of the founders of quantum theory, pondered on the problem in the tantalising epilogue of his classic What is Life? "Consciousness is never experienced in the plural, only in the singular. Even in the pathological cases of split consciousness or double personality the two persons alternate, they are never manifest simultaneously. In a dream we do perform several characters at the same time, but not indiscriminately: we are one of them; in him we act and speak directly, while we often eagerly await the answer or response of another person, unaware of the fact that it is we who control his movements and his speech just as much as our own." Schrodinger encapsulated the problem of consciousness in the form of two premises: "(i) My body functions as a pure mechanism according to the laws of nature.

(ii) Yet I know, by incontrovertible direct experience, that I am directing its motions, of which I foresee the effects, that may be fateful and all-important, in which case I feel and take full responsibility for them."

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To avoid a contradiction here, he said, "The only possible inference from these two facts is, I think, that I - I in the widest meaning of the word, that is to say, every conscious mind that has ever said or felt 'I' - am the person, if any, who controls the 'motion of the atoms' according to the laws of nature." And this would lead you to say, Schrodinger provocatively suggested: "Hence I am God Almighty."

Though even today to many western ears such a statement sounds both "blasphemous and lunatic"-and in 1943 it caused the rejection of What is Life? by its original (Catholic) publisher - the idea is hardly new. As its author noted, this "grandest of all thoughts" was recorded in the Upanishads more than 2,500 years ago, and has long been considered the deepest insight in Indian philosophy. Surely, said Schrodinger, the singularity of consciousness is more intuitively convincing than the western idea of a plurality of consciousnesses, which leads inevitably to the invention of souls - as many as there are bodies - and to unhelpful questions such as whether the soul survives death and whether animals (and bacteria) have souls? Towards the end of his life Schrodinger stated: "The world is a construct of our sensations, perceptions and memories. It is convenient to regard it as existing objectively on its own. But it certainly does not become manifest by its mere existence."

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His friend and scientific colleague Albert Einstein could never bring himself to agree (and thus could never accept that quantum theory was the fundamental description of nature). Nature, for Einstein, had to be independent of human consciousness. In 1930, arguing with the Indian poet Rabindranath Tagore, Einstein stated: "Man defends himself from being regarded as an impotent object in the course of the universe. But should the lawfulness of events, such as unveils itself more or less clearly in inorganic nature, cease to function in front of the activities in our brain?

"Leaving aside the inconsistency of such a view, the influence of alcohol and other sharply controllable factors on our thoughts, feelings and activities should show very distinctly that determinism does not stop before the majesty of our human will.

"Maybe, we and human society require the illusion of freedom in our human activities! I I believe that whatever we do or live for has its causality; it is good, however, that we cannot look through it."

The nature of causality, determinism and free will continue to underlie the burgeoning scientific debate about the nature of consciousness. As Alwyn Scott wrote in 1995 in his Stairway to the Mind, any physicist who chose to tell a major scientific meeting that he believed in an omniscient God would most likely be written off as a "misguided fundamentalist" - if instead he were to profess belief in a Theory of Everything that determines every fact of the future from the facts of the past, many would welcome him. But what is new and exciting in the 1990s, is that through technological advances in many fields - bringing a vastly increased sensitivity and diversity of technique to the study of the brain - science is at last becoming capable of investigating old questions about mind and brain empirically. As a result, physicists, mathematicians, computer scientists, chemists, biologists, geneticists, psychologists, psychiatrists, philosophers, linguists, anthropologists, theologians and others, even mystics, are now listening to each other with renewed interest. As the articles and reviews in this special issue demonstrate, science cannot yet encompass the mystery of our introspections, but it is beginning to move in that direction.

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