The distance between a neurone and a human mind seems very great, and to many philosophers and scientists quite impossible for science to cross. Even if minds are made from brains, and brains are made from billions of neurones, there seems no way to get from one sort of thing to the other.
Nicholas Humphrey's whole life as a scientist has been spent on that journey: in the 1960s he was part of the first team to discover how to record the activity of single neurones in a monkey's visual cortex; nearly 40 years later, he has reached a grand theory of how consciousness might have arisen in a Darwinian world, and why it might give us reasons to live.
The journey has been like the path of a neurone, full of twists and branchings and decisive contacts that altered its course. He has worked with monkeys in laboratories and in the wild. He has been a media don, a campaigner against nuclear weapons and the holder of a chair in parapsychological research who was dedicated to debunking even the possibility of telepathy or survival after death. He is an atheist, and the man who suggested to Richard Dawkins the analogy of viruses of the mind for religions; yet nowadays he talks as if spirituality were the thing that makes us human.
There is a self-confidence to this rather headlong life which stems, he thinks, in part from his background in the aristocracy of Cambridge. His father was an immunologist and FRS, his mother a psychiatrist and niece of John Maynard Keynes. In all, six of his relatives were fellows of the Royal Society, and one of his grandfathers, AV Hill, had won a Nobel prize. He never doubted he wanted to be a scientist: "It was what everyone around me was doing; the idea that I could have been professional at any other thing never really crossed my mind. I have to say there was a certain snobbishness about our attitudes. Anyone who didn't live in a large house didn't really count. Anyone who didn't have 15 cousins didn't count, and anyone who didn't have tea with a Nobel-winning grandpa wasn't really worth talking to either."
This sounds arrogant, but it is arrogance recollected after chastening. His career, which started out with great promise, has not run entirely smoothly. At first he wanted to be a physicist. At Westminster School, where he was educated, there was an inspired science teacher who devised a way for his pupils to measure the speed of light as it travelled the length of a London street and back. But when Humphrey went up to Trinity in 1961 on a scholarship to read mathematics and physics, he was disappointed in the course. He began to be fascinated by biology instead.
Though to many scientists biology feels messy and incoherent, to Humphrey it was much more logical and elegant than chemistry or physics: "Once I got into biology my eyes were open to a world of phenomena, a world of explanations, which had a kind of perfection I hadn't found before. There is no unifying theory in chemistry like evolutionary explanations in biology." As an ambitious young man, he set his sights on the biggest biological mystery he could find - human consciousness - so he switched to psychology, and began to work with monkeys under Larry Weiskrantz.
Humphrey was part of the team that first discovered how to record the activity of single nerve cells in a monkey's brain. Two other members later got Nobel prizes for this work, which underlies an enormous amount of subsequent research, since it made it possible to trace the ways in which the visual cortex receives and processes signals from the eyes. It was known in principle what was happening, but now the exact brain cells involved in image processing could be found and monitored.
His next discovery was wholly unexpected and is still hard to believe. In the laboratory was a monkey named Helen, who had been blinded when her visual cortex was cut with a scalpel. Humphrey decided to see what contact he could establish with the monkey, and got enough reaction to keep going. Over a period of seven years, he managed to coax out a sort of sense of sight. He played with the monkey, took her for walks, and did everything to persuade her that she could see: "Through this very intense and personal relationship - daily wondering what it was like to be her, and trying to get inside her mind - I began to get, I think, some insights into the general nature of consciousness. "It was like being part of a miracle. It wasn't really as if I had touched her with a healing hand, and made the blind see, but there are all those parables and models - and it was a bit like that."
Even four decades later, his excitement and pain are evident when he thinks of this. "It was a very sad moment when the monkey was killed. Of course she had to be. It was very important to know exactly what the lesion was. So [they] did it while I was away. I found it quite disturbing, though I think the research was interesting and important ... I wouldn't want to criticise anyone else who'd want to do it."
His next project was even more ambitious: to work on the aesthetic senses of a monkey. "It wasn't - not exactly - to make amends, but something like that was on my mind when I decided to work on aesthetics. I thought I would find out what monkeys would like doing if they had the choice."
This work was, very largely, a failure. He found that monkeys were strongly affected by colour, but shapes and sounds meant little to them. His first marriage was breaking up (he is now married, with two children, to an American psychologist), so in 1972 he went off to Rwanda for three months, to study mountain gorillas with Dian Fossey. Again, the question of what made us different arose: what had been the spur, or the reward, for human evolution, for our language and our consciousness. The answer he then came up with has been very influential. Variously known as "Machiavellian" or "social" intelligence, it is the idea that our brains evolved to cope not with the world around us, but with the people - or proto-people - of our ancestors' social groups.
Consciousness, in this theory, is a knowledge of what is going on in our own minds, and we have it so that we can better understand what is going on in the minds of those around us, so that we can manipulate them and avoid being manipulated in our turn. This fits human consciousness into a normal biological framework: it offers the possessor of bigger and better brains the kind of advantage that natural selection can see and work on.
For most of the 20th century consciousness had been out of bounds for scientists, and even for behavioural psychologists. Humphrey's original theory was one of the first signs that it could become a legitimate and fruitful area of scientific study. By the late 1970s he was a rather glamorous figure, living with the actress Susannah York, agitating against nuclear weapons - "We were always up on plinths in Trafalgar Square" - and in 1982 he was invited by Channel 4 to write and present a 10-part series on his theory. So he asked for leave of absence from the university and, when it was refused, resigned to make the programmes.
"I have tended to think that life's there as an exploration - don't pass up opportunities, whatever they are - and to have a certain sense that I'll be OK. At certain points I haven't. I've taken risks and then I'm very nearly not OK." He likes to quote Lord Byron: "The great object of life is sensation - to feel that we exist, even though in pain."
When the television series was finished, he could not get another academic job in England. Margaret Thatcher had come to power and the universities were shrinking. He was rescued by his friend Daniel Dennett, who found him a job at Tufts University, near Boston, and the two men worked closely together for years. In the mid-1990s he was able to move back to Cambridge, to a chair devoted to parapsychological research: since the whole burden of his interest in the subject was that he did not believe in it, he wrote Soul Searching, a book arguing that telepathy must be in principle impossible, and that Jesus was a conjuring charlatan like Uri Geller.
Yet, at the same time, he was developing a new and more complex theory of consciousness, which puts something like the soul at the centre of human existence. In his new theory the clue to the "hard problem" of consciousness - the problem of why and how minds appear from matter - is attacked head-on. The fact that we find it so difficult and so threatening to believe, as he says, "that there is nothing more to human experience than the churning of chemicals and electrons within the brain" seems to him to contain the kernel of the solution to the hard problem. If it is so difficult for us to think that way, then the difficulty might in some sense have been designed by natural selection.
Human beings, he writes, "have a self that seems to inhabit a separate universe of spiritual being. As the subjects of something so mysterious and strange, we humans gain new confidence and interest in our own survival, a new interest in other people, too. This feeds right back to our biological fitness, in both obvious and subtle ways. It makes us more fascinating and more fascinated, more determined to pursue lives wherever they will take us. In short, more like the amazing piece of work that humans are."
The theory is, like every other theory of consciousness, extremely controversial. After 200 years in which science has appeared to dethrone God and deny the possibility of the soul, Humphrey is the first man to claim that science can agree that we have souls - but that it was natural selection, not God, which gave us them.