I say all this just to demonstrate the formidable power of the brain's simulation software. It is well capable of constructing 'visions' and 'visitations' of the utmost veridical power. To simulate a ghost or an angel or a Virgin Mary would be child's play to software of this sophistication. And the same thing works for hearing. When we hear a sound, it is not faithfully transported up the auditory nerve and relayed to the brain as if by a high-fidelity Bang and Olufsen. As with vision, the brain constructs a sound model, based upon continuously updated auditory nerve data. That is why we hear a trumpet blast as a single note, rather than as the composite of pure-tone harmonics that gives it its brassy snarl. A clarinet playing the same note sounds 'woody', and an oboe sounds 'reedy', because of different balances of harmonics. If you carefully manipulate a sound synthesizer to bring in the separate harmonics one by one, the brain hears them as a combination of pure tones for a short while, until its simulation software 'gets it', and from then on we experience only a single note of pure trumpet or oboe or whatever it is. The vowels and consonants of speech are constructed in the brain in the same kind of way, and so, at another level, are higher-order phonemes and words.

Once, as a child, I heard a ghost: a male voice murmuring, as if in recitation or prayer. I could almost, but not quite, make out the words, which seemed to have a serious, solemn timbre. I had been told stories of priest holes in ancient houses, and I was a little frightened. But I got out of bed and crept up on the source of the sound. As I got closer, it grew louder, and then suddenly it 'flipped' inside my head. I was now close enough to discern what it really was. The wind, gusting through the keyhole, was creating sounds which the simulation software in my brain had used to construct a model of male speech, solemnly intoned. Had I been a more impressionable child, it is possible that I would have 'heard' not just unintelligible speech but particular words and even sentences. And had I been both impressionable and religiously brought up, I wonder what words the wind might have spoken.

On another occasion, when I was about the same age, I saw a giant round face gazing, with unspeakable malevolence, out through the window of an otherwise ordinary house in a seaside village. In trepidation, I approached until I was close enough to see what it really was: just a vaguely face-like pattern created by the chance fall of the curtains. The face itself, and its evil mien, had been constructed in my fearful child's brain. On 11 September 2001, pious people thought they saw the face of Satan in the smoke rising from the Twin Towers: a superstition backed by a photograph which was published on the Internet and widely circulated.

Constructing models is something the human brain is very good at. When we are asleep it is called dreaming; when we are awake we call it imagination or, when it is exceptionally vivid, hallucination. As Chapter 10 will show, children who have 'imaginary friends' sometimes see them clearly, exactly as if they were real. If we are gullible, we don't recognize hallucination or lucid dreaming for what it is and we claim to have seen or heard a ghost; or an angel; or God; or — especially if we happen to be young, female and Catholic — the Virgin Mary. Such visions and manifestations are certainly not good grounds for believing that ghosts or angels, gods or virgins, are actually there.

On the face of it mass visions, such as the report that seventy thousand pilgrims at Fatima in Portugal in 1917 saw the sun 'tear itself from the heavens and come crashing down upon the multitude',⁴⁹ are harder to write off. It is not easy to explain how seventy thousand people could share the same hallucination. But it is even harder to accept that it really happened without the rest of the world, outside Fatima, seeing it too — and not just seeing it, but feeling it as the catastrophic destruction of the solar system, including acceleration forces sufficient to hurl everybody into space. David Hume's pithy test for a miracle comes irresistibly to mind: 'No testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous than the fact which it endeavours to establish.'

It may seem improbable that seventy thousand people could simultaneously be deluded, or could simultaneously collude in a mass lie. Or that history is mistaken in recording that seventy thousand people claimed to see the sun dance. Or that they all simultaneously saw a mirage (they had been persuaded to stare at the sun, which can't have done much for their eyesight). But any of those apparent improbabilities is far more probable than the alternative: that the Earth was suddenly yanked sideways in its orbit, and the solar system destroyed, with nobody outside Fatima noticing. I mean, Portugal is not that isolated.*)

That is really all that needs to be said about personal 'experiences' of gods or other religious phenomena. If you've had such an experience, you may well find yourself believing firmly that it was real. But don't expect the rest of us to take your word for it, especially if we have the slightest familiarity with the brain and its powerful workings.

There are still some people who are persuaded by scriptural evidence to believe in God. A common argument, attributed among others to C. S. Lewis (who should have known better), states that, since Jesus claimed to be the Son of God, he must have been either right or else insane or a liar: 'Mad, Bad or God'. Or, with artless alliteration, 'Lunatic, Liar or Lord'. The historical evidence that Jesus claimed any sort of divine status is minimal. But even if that evidence were good, the trilemma on offer would be ludicrously inadequate. A fourth possibility, almost too obvious to need mentioning, is that Jesus was honestly mistaken. Plenty of people are. In any case, as I said, there is no good historical evidence that he ever thought he was divine.

The fact that something is written down is persuasive to people not used to asking questions like: 'Who wrote it, and when?' 'How did they know what to write?' 'Did they, in their time, really mean what we, in our time, understand them to be saying?' 'Were they unbiased observers, or did they have an agenda that coloured their writing?' Ever since the nineteenth century, scholarly theologians have made an overwhelming case that the gospels are not reliable accounts of what happened in the history of the real world. All were written long after the death of Jesus, and also after the epistles of Paul, which mention almost none of the alleged facts of Jesus' life. All were then copied and recopied, through many different 'Chinese Whispers generations' (see Chapter 5) by fallible scribes who, in any case, had their own religious agendas.

A good example of the colouring by religious agendas is the whole heart-warming legend of Jesus' birth in Bethlehem, followed by Herod's massacre of the innocents. When the gospels were written, many years after Jesus' death, nobody knew where he was born. But an Old Testament prophecy (Micah 5:2) had led Jews to expect that the long-awaited Messiah would be born in Bethlehem. In the light of this prophecy, John's gospel specifically remarks that his followers were surprised that he was not born in Bethlehem: 'Others said, This is the Christ. But some said, Shall Christ come out of Galilee? Hath not the scripture said, That Christ cometh of the seed of David, and out of the town of Bethlehem, where David was?'

Matthew and Luke handle the problem differently, by deciding that Jesus must have been born in Bethlehem after all. But they get him there by different routes. Matthew has Mary and Joseph in Bethlehem all along, moving to Nazareth only long after the birth of Jesus, on their return from Egypt where they fled from King Herod and the massacre of the innocents. Luke, by contrast, acknowledges that Mary and Joseph lived in Nazareth before Jesus was born. So how to get them to Bethlehem at the crucial moment, in order to fulfil the prophecy? Luke says that, in the time when Cyrenius (Quirinius) was governor of Syria, Caesar Augustus decreed a census for taxation purposes, and everybody had to go 'to his own city'. Joseph was 'of the house and lineage of David' and therefore he had to go to 'the city of David, which is called Bethlehem'. That must have seemed like a good solution. Except that historically it is complete nonsense, as A. N. Wilson in Jesus and Robin Lane Fox in The Unauthorized Version (among others) have pointed out. David, if he existed, lived nearly a thousand years before Mary and Joseph. Why on earth would the Romans have required Joseph to go to the city where a remote ancestor had lived a millennium earlier? It is as though I were required to specify, say, Ashby-de-la-Zouch as my home town on a census form, if it happened that I could trace my ancestry back to the Seigneur de Dakeyne, who came over with William the Conqueror and settled there.