Something unique in art happens when painters are their own subjects. But what? Tom Lubbock ponders truth and reality on the eve of an exhibition of self-portraits

The date was 1917. The place was the Omega Workshops, the London centre for advanced art. The witness to this private performance was the painter Winifred Gill, lying unnoticed on a bed, in a shadowy corner of a studio, pretending to be fast asleep as the scene unfolded before her narrowed eyes. And the man himself? It is the artist Percy Wyndham Lewis, leading light of British modernism, polishing his public image in the glass.

It could be any man, or any woman, no? This story about Wyndham Lewis is funny, but doesn't seem to reveal someone of excessive self-regard or calculation. Everyone rehearses themselves, prepares a face for the world. Everyone indulges in this kind of image-making, every morning, everyday.

Yes, but with this difference: Wyndham Lewis had an extra option. He could also "do" himself in paint on canvas. If he wanted to shape the face he showed to the world, if he wanted to make his self-image public, he could realise this desire in a more fixed and lasting form. He was an artist, after all.

It's an obvious but also an odd fact that all self-portraits are self-portraits of artists. There are portraits of many kinds of people, but the special me-on-me perspective that self-portraiture offers is restricted to that small class of people who can cut it with a brush.

Passing along the parade of pictures hanging in Self-Portrait: Renaissance to Contemporary, the exhibition that opens at the National Portrait Gallery in London next week, you'll see 55 human images, made between 1433 and 2005, each one showing you how some man or some woman saw themselves. But among them, you'll find no kings or queens, no philosophers or novelists or composers, no philanthropists or criminals. Every single one is a painter: Rembrandt, Van Gogh, Kahlo, Reynolds, Velazquez, Artemisia - sadly not Wyndham Lewis, but all the usual suspects. All artists.

It seems an arbitrary limitation on our knowledge. How someone feels about how they look, literally how someone sees themselves - this is a central dimension of their life. And if we could see it, it would tell us a lot. We have one or two slightly dodgy portraits of Shakespeare, but who wouldn't like to have Shakespeare's self-portrait? Or Hitler's? Or Freud's? Or Margaret Thatcher's? If you're interested in these personalities, then to know the kind of image they'd have made of themselves, would be a Holy Grail.

Anybody (practically) can write an autobiography that gives you roughly their verbal view of themselves. But not anybody can paint themselves, even when they try. There are self-portraits by DH Lawrence, for example, but they tell you nothing, they're just not good enough. Likewise the composer Arnold Schoenberg. The young "artistic" Hitler, too, must have done some. They haven't turned up, and probably wouldn't tell much if they did. You have to be pretty good to get anything across. And no doubt there's little to be done about this. But it still feels like discrimination. We all - if we have eyes - look in the mirror. We all have selves and self-images. We want to tell the world. We have a right to self-portraiture.

But examine this desire. Ask: what is it we get from self-portraits? Why do we specially value them? What do they give us that plain portraiture does not?

They're actually a pretty mixed bunch, self-portraits. They come across in all sorts of ways. Some are arrogantly doomy, like Salvator Rosa's self-portrait as a black mountain, his upper body a dark triangle silhouetted against a sky, his baleful expression almost lost in shadow. Some are unbelievably vain, like Lovis Corinth's self-portrait as a sex-god, a powerful figure, in a loose shirt, with brushes and palette in hand and naked woman pressing her body against his breast.

The self-portrait can be wretched, like Victor Emil Janssen, stripped to the waist, baring his skinny consumptive torso. It can be firmly dashing, like Rubens. It can be casually cheery, like Judith Leyster, swinging round from her easel with a twinkle. There may be an element of self-mockery, as in Edgar Degas's view of himself as a rather stiff, proper young gent, doffing his hat; or of extravagant self-pity, as in Gustave Courbet's self-image as a soldier dying on a battlefield.

Self-portraiture seems to offer a history of vanity, showing us the changing postures which self-assertion takes. Pre-Romantic artists display their social standing. Post-Romantic artists display their psychic wounds. And more recent artists display a self that has moved beyond grasp. Francis Bacon morphs into a swerving road-accident of brush strokes. Gerhard Richter withdraws into tantalising lost focus. Chuck Close deconstructs into an ornamental pattern of pixels. That's how we like to see ourselves these days - fragmentary, unpindownable.

Of course, the path of history isn't so straight. Back in the 17th century, Cristofani Allori could portray himself as a severed head, dead, hanging by the hair from a woman's hand. Judith with the Head of Holofernes is the title. Judith is played by an ex-girlfriend. Not even Frida Kahlo went that far in the victimisation stakes. It sounds very 1990s, though.

So, evidently, we do not go to a self-portrait for accurate information. It's about the last place to look for that. The self-portrayer is always likely to be putting something over on us. And that of course is the point. Whatever the self-portrait tells you about its subject, it tells you something else too. Here's what this man or woman thought they were like. Here's how they wanted the world to see them.

The facts that a self-portrait presents may not be true. But the desires that a self-portrait reveals are. Corinth may or may not have been a sex-god, but he certainly imagined he was, and wanted others to think so. His self-portrait gives him away. That's the magic of self-portraiture. That's why we feel in special contact with the figures in these pictures. Their self-regard can't help showing through. And that's how we often judge self-portraits, by the kind of self-regard they declare.

We have a sharp eye for the boaster, the smoothy, the tart, the drama queen, the poser, the moaner, the champion self-deluder. We have a fond eye for the modest, the objective, the candid, the courageous. We quite like a little spontaneous vanity in a young person. The young Joshua Reynolds shows himself armed with a brush, poised on the back foot, like Jack the Giant Killer. Knowing humour goes down well, too. Sophonisba Anguissola gives herself and us a charming, anxious, clever, serious Jane Austen look. But the aging Rembrandt gets the biggest love. How heroically he accepts his poor old sad declining self. He's the model for all late self-portraits. It's how we should all face ourselves when we're on the way out.

Self-portraiture, for the viewer, is this particular way of looking at a human image. You read it as the subject's own self-assessment in paint. But just try an experiment. If you go to the Self-Portrait show, take a look also at a straight * * portrait - somewhere else in the National Portrait Gallery, or in the National Gallery next door - and look at it in exactly the same way. Pretend it is a self-portrait. Say to yourself: this figure in the picture, this is how she saw herself, this is how he wanted the world to see him. Strangely, the experiment works.

There is no visible distinction between a face portrayed and self portrayed. Someone facing a painter doesn't appear different from someone facing a mirror. Did you think there'd be some "I'm gazing at myself" look in the eyes of self-portraits, that straight portraits lack? No, you can easily interpret one as the other. You take the image that an artist has given to a sitter, and imagine it as the sitter's own self-image. Straight portraits can yield all the self-portrait feelings, the full range of frankness and phoneyness, arrogance and anxiety. (And now, for good measure, go back along the self-portraits, and read them all as portraits.)

This looks like an answer to the distribution problem. The self-portrait is unfairly restricted to artists? But only if you insist. You can expand massively the field of self-portraiture, simply by deciding to treat straight portraits - of which there are many, many more - as self-portraits, too. True, it's possible that (say) Ingres's massive portrait of Monsieur Bertin is not the self-portrait that this 19th-century Parisian gentleman would actually have painted of himself. But someone could have painted it as a self-portrait. The picture can work equally well as a study in third person character, or in first person self-regard. It can be imagined either way. It's a brilliant portrait and a brilliant self-portrait.

Hold on, though. Aren't there actually some differences? Surely self-portraiture has its tell-tale signs, its typical symptoms. Yes, it has some. The person in the picture is often seen at work on a picture, or shown holding a brush and palette, sometimes in the wrong hands. The person is looking straight out - or maybe not quite straight, but giving a sideways glance, with the face in three-quarter view, because for painting purposes they need to be able to flick their eyes between mirror and canvas, while holding their head steady. They may have a look of intense concentration. The frame of the mirror may appear in the picture. All these are indicators that you have a self-portrait before you.

But they're not reliable. There are self-portraits that don't have them. There are straight portraits that do. Look at Suzanne Valadon's The Blue Room, where a large woman in pyjamas is lolling on a bed, cigarette in mouth, not facing out. It's the artist, but if you didn't know, you'd never tell. Or look at Manet's portrait of the painter Eva Gonzales, seen at work on a flower painting, brush and palette in hand. It might well come over as a self-portrait if you didn't spot the style. Self-portraiture can move away from the mirror. Portraits have eye contact too.

We know the facts in those two cases. But in hundreds of others, we don't. Most picture galleries have a post-Renaissance image of a young man, whose author is unknown, and the picture might be called A Portrait of the Artist as a Young Man - or just A Portrait of a Youth. There's nothing to tell which it is, and all your looking for some peculiar self-consciousness in the eyes will yield nothing.

Actually, the first picture in this show is in that doubtful category. It's by Jan van Eyck, that's certain. But whether or not it's of Jan van Eyck is moot. A middle-aged man in a turban eyes the viewer steadily. There's some evidence that it could be a self-portrait. Of course, we would like the father of oil painting to have left us his own image. But we just don't know what he looked like. Portrait of a Man is what it remains.

Besides, the signs of self-portraiture can be simulated. And if we wanted to distribute self-portraiture more widely around the population, another thing we could do is invent a new kind of painting. Call it "the pseudo-self-portrait". It seems a very promising genre. It involves a special act of imagination. An artist paints a portrait of someone, but adds in some of the typical indicators of self-portraiture - painting tools, sidelong glance etc. Now, here's the good bit. The idea is to paint it as the artist imagines the sitter would have painted himself or herself. How would Freud have done it, or Hitler, if they'd had the knack? That's the job.

In fact, there are pictures like that, or very nearly. There's one in the show. It's a late-ish self-portrait of Rembrandt, the biggest name in this business. But it's not by Rembrandt. It's by one of his pupils, an artist in his studio (it isn't known which one). But it really is a Rembrandt self-portrait. It has the thick paintwork, the figure surfacing from a surrounding gloom, the face touched with faint light. It has the true Rembrandt look; worn, sorrowful, resigned, but bearing up.

Of course, this pupil knew what a Rembrandt self-portrait looked like. He'd seen plenty; he didn't work only from imagination. But it's more than that. The work may be inferior, but the impersonation is deep. It's not a copy or a pastiche. This painter knows what it feels like to be old Rembrandt painting himself.

What weird thing is this, the self-portrait done by someone else? You have to realise that Rembrandt really was a business. Rembrandt self-portraits were among the goods produced by his studio. There was a market for them. And if the master was busy, the pupils could do it. Does that seem wrong? Perhaps the customers were conned. Or perhaps they understood that someone's self, their own self-perception even, is not the absolutely private property that we often take it for. It wouldn't be wrong to include this picture in a show of Rembrandt self-portraits, even though it's not directly by him.

No, but how about Van Gogh? You can't imagine a Van Gogh self-portrait painted by someone else? Well, it has been imagined - as it happens, by Wyndham Lewis, painter and writer. In his novel The Revenge for Love, we meet another artist at a mirror, with a cap on his head and a stern look in his eye. "In the fake masterpiece factory at Shepherd's Bush, Stamp sat at his workstool easily, with limber grace - erect in the saddle. A frowning eye was fixed upon his image in the glass. He was disguised in the fur cap of a Canadian trapper. A heavy white bandage, descending under his chin, covered his right ear. He was supposed to be Van Gogh. He was engaged in the manufacture of a Van Gogh 'self-portrait'."

Ha, ha. It's Lewis's particular joke to make it a Van Gogh. By the 1930s, when the novel was written, Vincent had already become a holy martyr of sincerity. Of all self-portraits, what a hideous irony that it should be his that are mastered by the faker's art, a faker sitting there in an absurd disguise, and pretending to have lost his ear.

But really, the irony is not so hideous. It is a blessing that our perspectives are so versatile - that we can imagine ourselves and others from the inside and from the outside, that we can paint not just a fake but a true self-portrait of someone else, and that portraits and self-portraits can change places before our eyes.

Self-Portrait: Renaissance to Contemporary, National Portrait Gallery, London WC2, 20 October to 29 January 2006 (020-7312 2463;