There is an anecdote told about Pierre Bonnard in old age, when he was living in the South of France not far from his old friend and colleague Matisse, two years younger. One day, during their usual discussions on painting and the current state of the art, they began speculating on whom the greatest living painters were, bringing up in turn the claims of X, Y and Z before agreeing - sometimes reluctantly - that none of these ultimately qualified for the Throne.

"So in that case," Matisse argued, "you and I are probably the two best." Bonnard looked at him dubiously and shook his head: if that were really the truth, he said, then painting must be in a bad way. (Matisse appears to have taken no offence at the implication). During the 1950s a rather similar discussion took place between Balthus, now widely regarded as the greatest living painter, and the English critic David Sylvester. Sylvester remarked that he could not make up his mind on who the greatest 20th-century painter was, Matisse or Bonnard. Balthus replied coldly that of course Bonnard was the greater of the two and that if Sylvester could not see that, "then you and I have nothing more to say to one another".

Bonnard died 51 years ago, widowed, childless, lonely and rather out of season both emotionally and artistically. Picasso dismissed him as "a neo-impressionist, a decadent," and many or most of the younger artists followed his lead. To them, Bonnard was not a "modern" painter in any real or relevant sense, more an afterglow of Impressionism and of the Nineties during which he had seemed, briefly, to be one of the avantgarde. Instead, he was a man of the "bourgeois" epoch who depicted charming interiors, nudes in baths, elegant men and women mingling in gardens or sailing their yachts off the Cote d'Azur. In short, he was seen as a product of the opulent, self-satisfied, insulated society of the Belle Epoque, which had gone up in flames in two world wars.

Though Bonnard developed quietly over decades and refined his style to the very end, to most people he did not appear to have changed in essence during half a century of steady production. Cubism, Fauvism, Futurism, abstract art had burst upon the art world inside a decade, so that he and other turn-of-the-century Intimistes such as Vuillard acquired almost a maiden-aunt image. After the first World War came first Neo-Classicism, then Surrealism, with neither of which Bonnard had anything at all in common. He was never very popular with American buyers - an important consideration, since the great American collectors and curators were powerful and influential figures internationally from the 1930s onwards. Compared with the market demand for Picassos and the other leading figures of the School of Paris, the prices paid for his pictures remained modest for a long time.

However, he always kept a solid core of influential admirers, including Matisse who consistently rated him as a master; and earlier in his career both Renoir and Monet had given him personal encouragement (after Monet's death in 1926, Bonnard was chosen to be one of the pall-bearers at his funeral). Meanwhile certain gifted painters of a younger generation - including Balthus, as already mentioned - found that they could learn much from him and his reputation grew quietly underground. By the 1960s a definite Bonnardisme began to surface and was especially strong in England, where William Scott, Patrick Heron and many younger figures acknowledged his influence; in America, too, the generation of "Painterly Realists" grouped around Fairfield Porter paid homage to him and his old friend and contemporary, Vuillard.

Suddenly, Bonnard began to look more modern than many of the Modernists, and there was a growing recognition that recent art history needed to be rewritten in order to accommodate men who, like him, had deliberately bypassed most of the 20th-century revolutions rather than falling behind them. Certainly he could not be discussed in terms of Cubism, allegedly the foundation-stone of modern art. Could it be that the allegedly backward-looking Bonnard was instead facing into the future - that in fact he had been ahead of the posse?

One factor in this posthumous renewal of interest was the release of many works which had never been seen publicly before, because of a lawsuit taken by surviving relatives of Bonnard's wife, Marthe, after his death. These were kept under legal distraint for a long time, until the matter was settled. The richness and logic of his later development at last fell into focus, so that he was no longer judged largely on the strength of pictures painted at the turn of the century. The big 1966 exhibition at the Tate Gallery was a revelation to a whole new generation, who began to realise that Bonnard was not only a fine colourist, as they had known already, but a great master of the French School - one of the last, quite possibly. Also, the swing against abstraction was well under way and the more formalist, dogmatic sects of Modernism were increasingly under siege. Here, ready-made and complete, was a man whose large output offered young painters an object lesson in how to treat the human figure, especially the female nude (although in practice, I find, abstract painters often admire Bonnard even more than figurative ones do).

Bonnard was a very private man, who generally shunned publicity - though he gave occasional interviews - and steered inquisitive people away from his domestic life, which he closely guarded. Outwardly his career was uneventful and for most of it he lived with Marthe Boursin, whom he finally married when they were both into late middle age. His social background was haut-bourgeois - his father was a high-ranking public servant, with a country house in the Dauphine as well as the family home in Paris - and he began as a respectable law student until, against his parents' wishes, he switched horses and turned to art.

He was soon a figure in the Paris avant-garde, one of the so-called Nabi group which included Vuillard, Roussel, Maurice Denis etc, and became friendly with the coterie of intellectuals grouped around the magazine La Revue Blanche. He illustrated volumes of poetry, did theatre and poster designs, and mixed a good deal in left-wing and anarchist circles just as, 50 years later, many young writers and painters became communists or fellow-travellers. It was the epoch of the Dreyfus case which divided France - particularly the intellectuals - and Bonnard belonged to the pro-Dreyfus, leftist and anti-clerical camp. His meeting with Marthe (he saw her first in a Paris street and followed her like a sleepwalker) was the most fateful event of his life, since they were to stay together until her death. Bonnard was then in his mid-twenties while Marthe was probably about the same age, though she pretended to be still in her teens. From a petit-bourgeois, provincial background, she had fought with her family and run away to Paris. She was a small, shapely, slightly boyish woman with a face which was childishly round from a frontal view, rather birdlike when seen in profile, and with an odd, capricious, difficult, somewhat neurotic personality. Her hold on Bonnard was absolute, and it seems to have been primarily sexual - almost a classic case of love-hate, from which he never quite escaped although at intervals he tried to do so.

Until recently, the tensions of his private life were only vaguely known, although his friends were fully aware of these, since Marthe fought with and alienated many of them and in later life was largely responsible for the reclusive life she and her husband led at Le Cannet, near Nice. The many paintings he did of her in her bath, or at least in bathroom settings, were taken for many years as expressions of intimate, domestic eroticism, like so much in French art. They are not necessarily his best pictures, but they are probably his best known.

In fact Marthe was not posing nude merely for him, since she spent hours of every day bathing and washing compulsively. She was a hypochondriac, neurotically self-obsessed, though her endless washing of herself may have been more than a compulsive mental tic; it is now believed that she suffered from tubercular laryngitis, for which hydropathy was then recommended. As a result, Bonnard spent much of his time taking her from one spa town after another, and her emotional and other demands on him were draining. (Sometimes he sat up with her all night and was too tired to paint the next day).

Bonnard made at least one serious attempt to leave her. In his fifties, he became intimate with a young painter, Renee Monchaty, who in contrast with Marthe was tallish, blond and full-bodied, though she was less than half his age. Apparently they determined to marry (legally they could do so, since he had not yet gone through a marriage ceremony with Marthe) but at a late stage Bonnard felt that he could not, after all, abandon his companion of nearly 30 years. Renee, shattered by this apparent renunciation, broke down and shot herself; Marthe, raging with jealousy, made Bonnard destroy all the relics of his relationship with the younger woman, including various paintings and drawings. A few years later, in 1925, they were legally married. The second World War overtook them in old age and Bonnard's old friends began to die away - including Vuillard in 1940, apparently as a fugitive from the invading Germans. His old circle in Paris had included many Jews, several of whom suffered persecution or worse while Bernheim, his dealer, closed his gallery early in the war. Short of income through lack of sales, subject to wartime shortages and rationing, and with Allied bombs occasionally falling near them, they were old and isolated and already half-forgotten. Marthe died in 1942; Bonnard outlived the war but died early in 1947, painting resolutely up to the last weeks of his life. None of these emotional whirlpools figure much in books about him, even in the one written little more than a decade ago by his nephew, Claude Terrasse, called Bonnard at Le Cannet. It is a charming work, but the cupboard is kept firmly closed on any skeletons and the "idyllic" side of Bonnard's art and outlook is stressed throughout. And this is the view of him which has predominated until now - the heir to Renoir, Boucher and Fragonard, hedonistic and charming, the intimiste painter of flowers, nudes and boudoir domesticity, a tranquil world with few threatening shadows. He was seen as one of the pillars of the "Mediterranean" phase of French art which began with Cezanne and ended with the death of Picasso (though in fact Bonnard probably spent as much time painting in Normandy as he did in the South). Today it is his more monumental qualities which are increasingly admired, his subtle sense of structure and highly original angles of composition, his anticipation of the "all-over" quality of postwar abstract painting, as well as his acknowledged genius as a colourist and his unique feeling for light. What are still neglected are most of the psychological undertones and erotic Angst of his work, and the fact that Bonnard had his own demon to wrestle with in private. He came of age intellectually in the Symbolist Nineties, a period in which Woman the Temptress, or even Woman the Destroyer, had become an obsession with writers, artists and musicians (the biblical figures of Salome and Delilah recur again and again, in paintings, poems, operas and plays).

Much of the impetus for this sexual morbidity and introspection came from Northern Europe, particularly via Strindberg's plays and the paintings of Edvard Munch - an obvious influence on Bonnard's early work, some of which has a surprisingly Expressionist edge. The Parisian circles he moved in as a young man were impregnated with fashionable Scandinavian influences, especially in the theatre where Ibsen was worshipped as a god by "progressive" audiences. Perhaps it was the growing emancipation of women which encouraged this new consciousness of the war between the sexes, and of the mutual enmities which middle-class domesticity was supposed to have bridged over or reconciled. What seems indisputable is that both sexes were increasingly aware they faced each other across a psychic gulf, which mere physical possession could not bridge. Sexual love, it seemed, simultaneously alienated or isolated men and women while yoking them together. In that sense, Marthe may have been Bonnard's domestic muse and model but she was also his Dark Angel or femme fatale, to whom he was tied inseparably for nearly 50 years. His last self-portraits, painted after her death when he was nearing 80, represent a man who is now terminally alone and waiting for the end.