I once was the guest of the week on a British radio show called Desert Island Discs. You have to choose the eight records you would take with you if marooned on a desert island. Among my choices was 'Mache dich mein Herze rein' from Bach's St Matthew Passion. The interviewer was unable to understand how I could choose religious music without being religious. You might as well say, how can you enjoy Wuthering Heights when you know perfectly well that Cathy and Heathcliff never really existed?

But there is an additional point that I might have made, and which needs to be made whenever religion is given credit for, say, the Sistine Chapel or Raphael's Annunciation. Even great artists have to earn a living, and they will take commissions where they are to be had. I have no reason to doubt that Raphael and Michelangelo were Christians — it was pretty much the only option in their time — but the fact is almost incidental. Its enormous wealth had made the Church the dominant patron of the arts. If history had worked out differently, and Michelangelo had been commissioned to paint a ceiling for a giant Museum of Science, mightn't he have produced something at least as inspirational as the Sistine Chapel? How sad that we shall never hear Beethoven's Mesozoic Symphony, or Mozart's opera The Expanding Universe.

And what a shame that we are deprived of Haydn's Evolution Oratorio — but that does not stop us from enjoying his Creation. To approach the argument from the other side, what if, as my wife chillingly suggests to me, Shakespeare had been obliged to work to commissions from the Church? We'd surely have lost Hamlet, King Lear and Macbeth. And what would we have gained in return? Such stuff as dreams are made on? Dream on.

If there is a logical argument linking the existence of great art to the existence of God, it is not spelled out by its proponents. It is simply assumed to be self-evident, which it most certainly is not. Maybe it is to be seen as yet another version of the argument from design: Schubert's musical brain is a wonder of improbability, even more so than the vertebrate's eye. Or, more ignobly, perhaps it's a sort of jealousy of genius. How dare another human being make such beautiful music/poetry/art, when I can't? It must be God that did it.

One of the cleverer and more mature of my undergraduate contemporaries, who was deeply religious, went camping in the Scottish isles. In the middle of the night he and his girlfriend were woken in their tent by the voice of the devil — Satan himself; there could be no possible doubt: the voice was in every sense diabolical. My friend would never forget this horrifying experience, and it was one of the factors that later drove him to be ordained. My youthful self was impressed by his story, and I recounted it to a gathering of zoologists relaxing in the Rose and Crown Inn, Oxford. Two of them happened to be experienced ornithologists, and they roared with laughter. 'Manx Shearwater!' they shouted in delighted chorus. One of them added that the diabolical shrieks and cackles of this species have earned it, in various parts of the world and various languages, the local nickname 'Devil Bird'.

Many people believe in God because they believe they have seen a vision of him — or of an angel or a virgin in blue — with their own eyes. Or he speaks to them inside their heads. This argument from personal experience is the one that is most convincing to those who claim to have had one. But it is the least convincing to anyone else, and anyone knowledgeable about psychology.

You say you have experienced God directly? Well, some people have experienced a pink elephant, but that probably doesn't impress you. Peter Sutcliffe, the Yorkshire Ripper, distinctly heard the voice of Jesus telling him to kill women, and he was locked up for life. George W. Bush says that God told him to invade Iraq (a pity God didn't vouchsafe him a revelation that there were no weapons of mass destruction). Individuals in asylums think they are Napoleon or Charlie Chaplin, or that the entire world is conspiring against them, or that they can broadcast their thoughts into other people's heads. We humour them but don't take their internally revealed beliefs seriously, mostly because not many people share them. Religious experiences are different only in that the people who claim them are numerous. Sam Harris was not being overly cynical when he wrote, in The End of Faith:

I shall return to the subject of hallucinations in Chapter 10.

The human brain runs first-class simulation software. Our eyes don't present to our brains a faithful photograph of what is out there, or an accurate movie of what is going on through time. Our brains construct a continuously updated model: updated by coded pulses chattering along the optic nerve, but constructed nevertheless. Optical illusions are vivid reminders of this.⁴⁷ A major class of illusions, of which the Necker Cube is an example, arise because the sense data that the brain receives are compatible with two alternative models of reality. The brain, having no basis for choosing between them, alternates, and we experience a series of flips from one internal model to the other. The picture we are looking at appears, almost literally, to flip over and become something else.

The simulation software in the brain is especially adept at constructing faces and voices. I have on my windowsill a plastic mask of Einstein. When seen from the front, it looks like a solid face, not surprisingly. What is surprising is that, when seen from behind — the hollow side — it also looks like a solid face, and our perception of it is very odd indeed. As the viewer moves around, the face seems to follow — and not in the weak, unconvincing sense that the Mona Lisa's eyes are said to follow you. The hollow mask really really looks as though it is moving. People who haven't previously seen the illusion gasp with amazement. Even stranger, if the mask is mounted on a slowly rotating turntable, it appears to turn in the correct direction when you are looking at the solid side, but in the opposite direction when the hollow side comes into view. The result is that, when you watch the transition from one side to the other, the coming side appears to 'eat' the going side. It is a stunning illusion, well worth going to some trouble to see. Sometimes you can get surprisingly close to the hollow face and still not see that it is 'really' hollow. When you do see it, again there is a sudden flip, which may be reversible.

Why does it happen? There is no trick in the construction of the mask. Any hollow mask will do it. The trickery is all in the brain of the beholder. The internal simulating software receives data indicating the presence of a face, perhaps nothing more than a pair of eyes, a nose and a mouth in approximately the right places. Having received these sketchy clues, the brain does the rest. The face simulation software kicks into action and it constructs a fully solid model of a face, even though the reality presented to the eyes is a hollow mask. The illusion of rotation in the wrong direction comes about because (it's quite hard, but if you think it through carefully you will confirm it) reverse rotation is the only way to make sense of the optical data when a hollow mask rotates while being perceived to be a solid mask.⁴⁸ It is like the illusion of a rotating radar dish that you sometimes see at airports. Until the brain flips to the correct model of the radar dish, an incorrect model is seen rotating in the wrong direction but in a weirdly cockeyed way.