Georges Braque is the third man of modern art. Matisse and Picasso rule over all. Braque stands in the shadows, as if draped in his own colours: Braque-brown, green, black. It was said of his hero Cézanne that he did not paint "Look at me" but "Here it is". So it was with Braque, who thought nothing of meditating on a painting for 10 years or more. His boldness was as deceptive as his slowness. At the official opening of the exhibition Picasso and Braque: Pioneering Cubism at the Museum of Modern Art in New York, in 1989, members of the Picasso family could be heard asking: "But where is Braque?"
He is resistant to the stock assimilations. This is partly a matter of the company he kept and partly the distance he travelled. Late in 1907 Braque embarked on a serial treatment of a little place called L'Estaque in the south of France (its houses, trees, roads, viaduct), which evolved over the next few months into a style of painting that had no name, no school, no code and no precedent - painting that left his peers breathless and his critics stunned.
The L'Estaque landscapes were not so much traditionally conceived landscapes as variations on the idea of landscape. Each element nested in context; but the sense of place was increasingly evanescent. Space was reconceived and reconstructed. Instead of receding tidily into the background, as prescribed by traditional perspective, the forms in Braque's paintings advanced disconcertingly towards the viewer. Landscapes became landslips, as though bearing witness to some underground volcanic activity or process of erosion. Houses and trees asserted themselves, in their quiet way, reclaiming inalienable rights to light, space and a certain dignity.
Contemplating the works, Matisse observed that Braque's houses (or the signs representing them) were employed formally, "to let them stand out in the ensemble of the landscape", and at the same time morally, "to develop the idea of humanity, which they stood for". In the beginning there was still a vestige of sky. The following summer the sky disappeared altogether, in favour of an "all-over" background with pictorial protrusions. Traditional landscape space met the same fate as traditional perspective. The sky was not seen again for 20 years.
Keen to make the strongest possible showing at the 1908 Salon d'Automne in Paris, Braque submitted six of his L'Estaque landscapes. On the jury were Guérin, Marquet, Matisse and Rouault. They rejected his whole submission. In such cases, the rules of the Salon allowed each member of the jury to "retrieve" one rejected work; Guérin and Marquet alone elected to do so, thus keeping two in play. This was too much for Braque, who withdrew them all and laid the blame at Matisse's door. The critic Louis Vauxcelles later recounted how Matisse told him: "Braque has just sent in a painting made of little cubes." The painting in question was almost certainly Houses at L'Estaque (1908), as yet unexhibited but neither unseen nor unheard.
After the dust had settled, Matisse remembered it as "really the first picture constituting the origin of cubism... We considered it as something quite new, about which there were many discussions." Later still, he let slip a further significant detail. He had seen it not in Braque's studio, but "in the studio of Picasso, who discussed it with his friends". Picasso had borrowed it to study and to learn. Matisse and Picasso knew well enough that there was a third man in town - a new gun - slower, perhaps, but dangerous.
Braque's rebellion had shattering implications. The audacity of this work, and the cool effrontery of its maker, propelled him into partnership with Picasso. He was the only artist ever to sustain such a relationship with that ravenous genius. They entered into an intense collaboration - at times almost cohabitation - legislating the future, vandalising the past, subverting western ways of seeing. The relationship lasted at least six years (1908 to 1914) on what might be called the conjugal model, and the rest of their lives (another 50 years) in remission. Its nature is not well understood: it was longer, more complex, more intimate and more evenly balanced than is often supposed.
In one way or another, Picasso's famous riff on Braque as his wife or ex-wife ("the woman who loved me best") has set the tone for much subsequent commentary. But Picasso should not be taken too literally. Many of his most celebrated remarks had a convenient reversibility. On one occasion when Braque was in hospital, Picasso was refused permission to see him. "The nurse wouldn't let me into his room," he fumed. "She said Madame Braque was with him. She didn't realise that I am Madame Braque."
It was Picasso who pursued Braque - more assiduously than any woman - and not the other way around. "Why doesn't Braque come and join me?" he asked a friend, plaintively, in 1961. "I always keep a floor for him." Picasso's needs were visceral and overwhelming. His encounter with Braque was in some measure the most satisfying relationship of his life. Later on there were jealousies, and callouses, and barbed bons mots on both sides. "It's well hung," said Picasso, of an exhibition of Braque's paintings. "It's well cooked," said Braque, of an exhibition of Picasso's ceramics.
Picasso was jealous of the marches Braque had stolen - Fruit Dish and Glass (1912), the first papier collé (pasted paper or collage), complete with mock wood wallpaper purchased in a shop in Avignon while Picasso was away in Paris - and of their claim on posterity. He was jealous of the tricks Braque continued to turn. Whenever Picasso visited the connoisseur Douglas Cooper he would go and study Studio VIII (1954-55), hanging in pride of place above Cooper's bed. Asked what he thought, he would mutter to himself: "Don't understand, don't understand."
Braque's mysterious Studios (1949-56), the refuge of all things and all notions, challenged Picasso to raise his game. He responded, indirectly, in the first of his variations on the most celebrated of all studios, Velázquez's Las Meninas, in 1957. But he could not match the elemental strangeness, the fathomlessness, the total envelopment achieved by Braque in those hallucinatory canvases. Late Braque is work of supreme distinction, to rank with the late work of Bach or Rembrandt or Rilke, but hardly yet assimilated, either to the painter's own career or to the common stock of cultural reference.
Ultimately, Picasso was jealous of the man himself. That Braque could do what he could not was a puzzle and a provocation. That Braque could be what he could not was a regular torment. Morally and metaphysically, Braque had a stature that was beyond rivalry, unapproachable. Braque invested heavily in being. "Few people can say: I am here," he reflected wisely. "They look for themselves in the past and see themselves in the future." He had an almost mystical aura about him, but he was the least delusional of men. He became le patron - detached, seclusive, inscrutable - master of the artless art. Tales of the mature Braque have a Zen-like ring to them. He was always suspicious of "talent" and insisted he had none. His dedication was complete. "Painting is an answer to everything," he told Nicolas de Staël, "even success."
For other painters, young and old, Braque was the painter. "After him, who?" asked Amédée Ozenfant. He was not merely influential, he had a much rarer distinction. He won for himself the kind of glory beautifully described by Paul Valéry: "To become for someone else an example of the dedicated life, being secretly invoked, pictured and placed by a stranger in a sanctum of his own thoughts, so as to serve him as a witness, a judge, a father, a hallowed mentor." Picasso would not be Picasso without Georges Braque. Many others felt the same. The third man set the tone. He has been cast as the great corroborator. In the end he corroborates no one but himself. It is time he came into his own.
· Alex Danchev will be giving a talk, "Who is Georges Braque?" at Tate Modern, London SE1, on June 3. His biography, Georges Braque, is published by Hamish Hamilton on May 26, price £25.