ART / Moving pictures: 'No other modern artist has ever made paint look quite so unlike itself, so possessed with living qualities.' Andrew Graham-Dixon on Bonnard

The view from the bedroom window of his villa at Le Bosquet, on a summer's day in 1946, was the pretext for one of the smallest and most glowing of Pierre Bonnard's late pictures. The artist looked out, perhaps through eyes half closed against the summer glare, and he saw an orange tree and an almond tree and a thick mass of green and yellow vegetation. He saw the tiles on the roof of a small jerry-built outhouse shining with orange vividness in the heat of the sun. He saw the humid thickness of the air.

But as Bonnard worked on The Little Window (View from the Painter's Bedroom), he did not really paint those things, or not those things alone. He painted the inexplicable intensity of his own feelings before them. His picture offers to the eye an image of the world transfigured. Using paint to create a gold-green sparkle shot through with passages of shadowy blue he made a surrogate for what he saw that seems tinged with the memories of other things seen. He might have been looking down, not across: not through a window at a landscape but through the floor of a glass-bottomed boat at fish and coral, gleaming and twinkling under the sea.

'Bonnard at Le Bosquet', a lovingly and sparingly selected display of the pictures which the artist painted during the last 20 years of his life at his villa in the South of France, is the most shiningly beautiful exhibition seen at the Hayward Gallery for many years. It is also confirmation that Bonnard was one of the great visionary painters of his century - and, indeed, of any other.

Bonnard never descended to the grandiose. He painted interiors. He painted views from windows. He painted still lifes, things on tables and shelves and in cupboards. He painted landscapes. He painted his wife, Marthe, and after her death he painted his memories of her. But out of these ordinary things he created pictures charged with glory and wonder.

A street near his home became The Pink Road, a mauve lake in which two diminutive figures and a dog have been marooned beside a house made of ice-cream with, beyond them, a skyline made of bright blue trees. The picture's oddity also strikes a chord of recognition, evoking the kind of day when heat and haze dissolve everything and when trees backlit by the sun do, indeed, look blue. The world through Bonnard's eyes may often seem to melt and deliquesce, to take on strange and apparitional qualities. But the nervous vitality of his touch and the trembling incandescence of his colour also represent truth to experience.

The nearly unreal vividness of this art is a form of insistence, Bonnard's way of emphasising that things did look this way to him. Objects in his paintings often seem more powerfully actual than merely real things, in the merely real world, do to most of us most of the time. Bonnard was a great painter of heat and light and of the heavy penumbra with which sunshine can surround things. The bowls and baskets of fruit that he depicted on tables in the sun are so simply painted that they could easily have seemed insubstantial. But they do not. Haloed by light, they are also anchored in reality - just as, transfigured by art, they are also weightily present. These pictures are Bonnard's sacraments.

He has been labelled an Intimist, but he was really a religious painter in a secular age: a man who saw the real world as a blessed place and recreated his perceptions of it in the form of painted worlds that have the quality of benedictions. There were not many visitors at the Hayward when I saw the exhibition but they were all smiling the same slightly stunned, beatific smile. They looked like people walking in sunshine for the first time in a long while, or the recipients of some sudden, unexpected, generous gift.

Bonnard's disposition was not uniformly or simply optimistic, and this show also contains pictures that speak of loss and melancholy and of the knowledge that life must come to an end. In his 1938 Self-Portrait (In the Mirror) Bonnard paints himself, with sad but unsentimental foreknowledge, as a quizzical slope-shouldered ghost. Bonnard's radiant epiphanies are themselves substantiated by the sense, within the mobile brushwork and evanescent colour that has been used to conjure them, of the transitoriness of those moments that they celebrate.

The mechanisms by which Bonnard invested his pictures with the quality of numinousness remain beyond articulation because they are not a matter of symbolism or allegory but of technique. There are, occasionally, hints of a leaning to mystical symbolism, of a kind, in his art. He depicts Marthe, the tutelary deity of so many of his pictures, in ways that make of her a mute goddess, an unspeaking figure with an aura about her of ancient myth. He makes much of doors and windows as framing devices, so they become metaphors for the way in which his is an art suspended at the threshold of the worldly and the otherworldly. But it is in the paint itself that Bonnard accomplishes the magical act of transformation that such small and understated pieces of symbolism announce as the ambitions behind his pictures.

No other modern artist - perhaps not even Matisse, although it is a form of heresy to say so - has ever made paint look quite so unlike itself, so nearly immaterial or so strangely possessed with living qualities, as Bonnard did. The trees in his landscapes catch fire, his skies heave, the air in his interiors pulses with mysterious energies. Inspect his pictures really closely and it is still not quite clear how the feat was managed: even from a few inches away, Bonnard's surfaces seem not like surfaces but like electrically charged ether or moving water. His tiny dabs and streaks and blotches of paint are almost alive and they seem to shift around as you look at them.

Bonnard is a much loved artist but the true magnitude of his achievement has been insufficiently recognised. He was too bourgeois to be great, it has been said. He did not paint the large themes of life, it has been said. But how inattentive and small-minded such objections are - and how irrelevant they are rendered by his finest work.

Take the tersely titled Nude in Bathtub, which Bonnard worked on for five years, between 1941 and 1946. It is a truly astonishing work, not least because in it we can see so clearly how Bonnard has, in his surreptitious and soft- spoken way, invented a whole new language of painting: a way of painting which is so mobile and fluctuant, in the shifts and moves of light and colour across its animated mosaic of a surface, that it becomes an almost tragically potent analogue for experience; a way of recreating, in the texture of art, the texture of life itself. It is an endless picture, such is the delicacy and hesitancy of the marks which form it, the way in which touch and colour and form bleed into and melt into one another. The wonky rectilinear grids of tiles on the wall and the floor of the bathroom contain, like a dim memory, recollections of the fractured world of the Cubists, but the Cubist project of attempting to picture the unstill and moving essence of things has been resumed in a painting that actually surpasses Cubist art in the accomplishment of the Cubist ambition. A room has become a world, a paradise sensorium of light and colour that celebrates what it can be to be alive but which also contains its own memento mori in the form of Marthe, lying in her bathtub like a body in a sarcophagus. This exhibition reminded me of the young Delacroix's description of how he felt when he saw Gericault's Raft of the Medusa for the first time. He tried to walk home but found himself instead running and whooping, like a madman, with involuntary and unconfined delight.

(Photograph omitted)