The Martians are always coming, according to Philip K Dick. The prolific and hugely talented science-fiction author was in a disturbed state of mind much of the time towards the end of his life, and by the time he died in 1982 - three months before the release of Ridley Scott's film Blade Runner, which was based on one of his stories - he was undoubtedly crazy. He believed that he was in touch with an extraterrestrial entity he called the Vast Active Living Intelligence System (or Valis), which was communicating with him via an earth-orbiting satellite. In a million-word journal, he recorded his belief that the world was still ruled by the Roman empire and detailed his experience of being under surveillance by the FBI and the KGB.
This is the stuff of paranoid delusion, yet Dick understood better than almost anyone the condition which led him and millions of others to believe that they had been in contact with alien intelligences. Such experiences, he believed, confirm the truth of "the Gnostic Gnosis: you are here in this world in a thrown condition, but you are not of this world".
Ever since the end of the Second World War, there have been sightings of objects flying across the sky in a way no human craft has been known to do, and in recent decades such experiences of extraterrestrial visitation have swelled into reports of alien abduction. Large numbers of seemingly unremarkable and well-balanced people claim to have been removed from the earth by aliens. It would seem that such a perplexing and large-scale phenomenon requires an explanation.
In Aliens: why they are here, Bryan Appleyard outlines three main ways of understanding these reports. The first, which he calls the "nuts and bolts" view, interprets them as accounts of events that actually happened; the spacecraft and their inhabitants are as real as the Tube we take to work, but have somehow escaped scientific observation. The psychosocial view, set out by Carl Jung among others, is that UFOs and their inhabitants are projections of our own minds. The last approach postulates a third realm distinct from our minds and the physical world, which is seen as the point of origin of the aliens who are reported to be visiting us.
Whether they are no-nonsense debun-kers, conspiracy theorists or believers in an occult third realm, those who seek to explain aliens try to reduce them to the terms of ordinary experience. For Appleyard, this gets things the wrong way round: what such phenomena reveal is the mysteriousness of human consciousness. Rather than trying to explain them away, he starts from the undoubted reality of aliens - as he puts it, they "may or may not exist, but they are all around us and they are trying to tell us something". In taking this line, he exposes himself to attack from scientific reductionists, who want an explanation that fits with what they think we understand already. Yet it seems to me that Appleyard's suspension of disbelief is the path of true inquiry. By leaving these anomalous experiences unexplained, he enables us to look at ordinary consciousness in a new light.
Aliens is an invigoratingly sceptical history of one of the most intriguing and important phenomena in postwar culture. It is endlessly fascinating, ranging from academic controversies - such as that surrounding the work of the late John Mack, the Harvard professor of psychiatry who lost favour with the university when he came to think that some reports of alien visitation might have some truth in them - to the prophets of artificial intelligence who predict the emergence of self-conscious mental life in machines. However, Appleyard does much more than offer a fresh take on late 20th-century cultural history. He interprets the postwar experience of alien visitation and abduction as a symptom of an ambivalence in human consciousness: a sense of being at once in and out of the world that seems to have existed as far back as we can go.
In the mid-18th century, the Swedish mystic Emanuel Swedenborg described travelling to other planets and meeting Martians, and there is a clear line of descent from the angels and demons encountered in medieval times to the close encounters of recent decades. Alien presences are recognised in virtually all cultures. It is only in the past hundred years or so that they have been ostracised as falling outside the dominant world-view.
Reports of aliens are disturbing because they remind us that our ordinary experience is in many ways highly anomalous. We tend to separate everyday perception from the altered states experienced in dreams, and maintain that the latter are human projections while the former represents the way things really are. The message of cognitive science is rather different. The world we perceive every day is also a human construct, far removed from "things in themselves". We live permanently in a virtual environment, and the question of what actually exists cannot be settled as easily as reductive materialists would have us believe.
Aliens are with us and they are not going away. There can be no adequate explanation of our encounters that does not recognise them as ultimately religious. The need for a connection with non-human things does not disappear when a secular world-view takes over. The sense of not belonging in the world persists in two seemingly contradictory ideas - in the humanist insistence that we are somehow special and unique, and in the belief that we are surrounded by alien intelligences. The question of whether aliens exist is a question about what human beings really are, and lies behind all contemporary debates in which hoary issues of science and religion are being forever rehashed. In Aliens, Appleyard confronts this question directly. The result is a profound meditation on what it means to be human.
John Gray is the author of Heresies: against progress and other illusions (Granta Books, £8.99)