Henri Cartier-Bresson, probably the century's greatest photographer, tells Andrew Robinson how chance, instinct and his training as a painter inform his art

Henri Cartier-Bresson is one of a handful of artists who deserve the label "living legend". Most of us know some of his photographs, whether or not we know who took them. There is the 1930s French family picnic on the banks of the Marne, seen from behind; the London crowd at the coronation of George VI, eager-eyed, while down below a man sleeps on the pavement in a scurf of newspaper headlines; the enigmatic smile of a wizened eunuch of the last emperor of China, taken in 1949; Henri Matisse surrounded by doves; Marilyn Monroe's fragile aura of beauty; and Mahatma Gandhi, seen from behind with his left hand splayed in sunlight, as if to say "What can I do?"- just before he was assassinated.

In the catalogue of Tete a Tete, an exhibition that opens at the National Portrait Gallery in London next month, art historian Ernst Gombrich compares Cartier-Bresson's photo-portraits to Old Masters like Rembrandt. Jean Clair, director of the Musee Picasso in Paris, introducing The Europeans, an exhibition of Cartier-Bresson's photographs of European life over more than half a century, at the Hayward Gallery, is even more laudatory: "As a writer, too bound to the earth, I envy Cartier-Bresson his being, in the manner of the angels, the messenger of the gods, someone who enables men to inhabit their own homeland."

Cartier-Bresson himself, who will be 90 in August - the ostensible reason for these exhibitions - is modest about his achievement. He tells me with a slightly exasperated shrug: "'The great photographer' - what does it mean? I'm just an ordinary human being." Others may regard him as the icon of 20th-century photography - no doubt including the academics who will speak on his work at a symposium at the Institut Francais next month - but the man himself says: "Celebrity is just like an obituary. There's only one thing that exists. It's the present and eternity."

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He is perhaps most famous for the fact that he hates being photographed, wishing to preserve his anonymity. When Oxford University gave him an honorary degree in 1975, the only photographer to receive this particular honour, he held a paper in front of his face to avoid the cameras. Though photographs of him do exist - including some taken by his photographer wife, Martine Franck (one of the few female members of Magnum, the world's leading photo agency, which Cartier-Bresson founded with three others in 1947) - they have usually been snapped without the subject's permission.

Almost equally famous is Cartier-Bresson's refusal to analyse his work. There is a classic image, for instance, which was taken behind the Gare St Lazare in Paris in 1932. It shows the silhouette of a man, fully clothed and wearing a hat, and his upside-down shadow, both slightly out of focus because he is in the act of stepping (leaping? - we are left to surmise) off a plank into an expanse of water. When, last autumn, the negative was reproduced in a Cartier-Bresson special issue of an American photographic magazine, a sharp-eyed reader noted that the image was frame number 39 - which, as even an amateur photographer knows, may or may not be a dud in a normal reel of 36. Anticipating what was about to happen, would it have been better for the photographer to have reloaded before shooting - and risked missing this unique image? "Did HCB know that this was the final frame?" asked the reader's letter. "Showing this particular image as negative number 39 powerfully illustrates 'the decisive moment' for me."

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This was a reference to the title of Cartier-Bresson's most celebrated book, Images a la Sauvette (The Decisive Moment), published in 1952, which played a key role in raising photography to the status of an art. His introduction has become a bible for students of photo-graphy, journalism and even film-making, not to mention historians of photography, of art and of European culture in the 20th century. In it Cartier-Bresson wrote: "To me, photography is the simultaneous recognition, in a fraction of a second,of the significance of an event as well as of a precise organisation of forms which give that event its proper expression."

When I tried to put the American reader's question to Cartier-Bresson, it produced one of his (also famous) explosions. He made a disgusted face and cried out, while gripping my arm with a force worthy of a man half his age,that this was just the kind of fatuous question that preoccupies far too many scholars, whether in photography or in any other academic field. And I remembered how often Cartier-Bresson has spoken of the importance of hasard - chance - in photography. No doubt he was lucky, on that day in 1932 behind the railway station, that his film did not run out at the wrong moment. But what had nothing to do with luck was the extraordinary fusion of significance and form perceived by the photographer's eye at "the decisive moment".

Nevertheless, Cartier-Bresson has encouraged certain writers, including scholars, who have wished to penetrate his mysteries. The fullest study is Henri Cartier-Bresson and the Artless Art by Jean-Pierre Montier, a young professor of literature at the University of Rennes. Here is how he and his subject met, according to Cartier-Bresson's characteristically quirky, hand-written preface to Montier's book.

A few years ago, on the intercom: 'Sir, I would like to write a thesis on your photographs.' / 'Thank you, sir, but there are more amusing ways of spending your time.'

I hang up. Some years later: 'Sir, I didn't take your advice and my thesis weighs more than two kilos.' / 'Come up.'

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By the time the book appeared in print in 1995, Cartier-Bresson had not only given Montier new statements about his work but had also approved the selection of photographs, making the book an extremely valuable visual record of "Cartier-Bresson by Cartier-Bresson".

Even so, Montier does not take us very much closer to the method by which Cartier-Bresson achieves his magic. The writer Dan Hofstadter, who spent many weeks with Cartier-Bresson and his wife and wrote probably the best account yet of his life and work (for The New Yorker, in 1989), once watched him discreetly size up a subject and then decide not, after all, to take a photo. Hofstadter tried to compare what he had observed with the approach of other photographers he knew, but realised he had learned nothing. "I saw that though some artists may pass through a mental terrain,leaving characteristic tracks, Cartier-Bresson does not."

Hofstadter concluded: "Nobody has ever figured out how he takes his incredible pictures, and many people make the mistake of thinking he knows.In fact, he doesn't; if he had a precise idea of what he was doing while he was doing it, he would probably be too slow to do it well." Or, as Cartier-Bresson informs me: "I am a hunter, who is a vegetarian."

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A vital clue to the process, because it differentiates Cartier-Bresson from most photographers, is his training as a painter and his friendships with artists such as Giacometti, Bonnard and Matisse, not to mention the director Jean Renoir, son of painter Auguste Renoir, for whom Cartier-Bresson worked as an assistant on Une Partie de Campagne and the classic film La Regle du Jeu.

There were painters in his family and he wanted to become one from a young age. In the late 1920s, he studied with the Surrealist Andre Lhote in Paris, and also in Cambridge. Though he destroyed most of these early works, a few survive. As do some paintings he did in 1943, after escaping from a German PoW camp and before joining the Resistance. (Another of his well-known photographs shows the chance exposing of a female Gestapo informer by a woman she had denounced.) Then, in the late 1960s, Cartier-Bresson, having left Magnum, gave up photography as a profession - he has since taken photos only when he feels like it - and took to painting and drawing. His first exhibition, in New York in 1975, was followed by many more; his latest show will open at the Royal College of Art next month. In 1989, he published a book of drawings, Line by Line.

As Cartier-Bresson admits, a little wryly, the natural reaction to a man's turning to painting late in life is: "It's his hobby." An old friend,the publisher Teriade (who was behind The Decisive Moment), advised him that he should sell his work, because no one would take him seriously as an artist unless he sold. Although the advice went against the grain - Cartier-Bresson has an aversion for the consumer society, sharpened by his experience as a photo-journalist in the 1940s, 50s and 60s, working for magazines like Life - Teriade turned out to be correct. At an exhibition of Cartier-Bresson drawings in London in 1996, prices were in the thousands.

An obvious question arises: how does his photography influence his drawing and painting, and vice versa? Cartier-Bresson believes emphatically that very few people use their eyes well, and that shamefully few photographers have studied composition as seriously as painters have. An intriguing point, highlighted by Gombrich in Tete a Tete, is that as a photo-portraitist Cartier-Bresson employs an infinite variety of angles and compositions; but when he handles a pencil, he does not use these compositional devices. "Here his searching eye and hand concentrate on the isolated head and its expressive features."

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Cartier-Bresson thinks: "Drawing is a meditation, fast or slow. With photography one is always on the crest of a wave, like a surfer, always struggling against time." Fundamentally, though, he sees no difference between photography and painting: "It is the same visual world. There is no aesthetic peculiar to photography or drawing. In both cases, there is a specific technique that is appropriate, which changes everything." His finest portrait drawings, often of his friends and family, certainly bear him out. As with all Cartier-Bresson's astonishing work, it is the sensibility behind the technique that is what really matters.

Andrew Robinson is literary editor, THES.