AT THE BEGINNING of Picasso's Mask, Andre Malraux is summoned by Picasso's widow, Jacqueline, to discuss the artist's estate and the placement of his collection of works by other artists. As they walk through the studio, turning around paintings by Picasso stacked against the walls, Malraux recalls his visits to the artist's studio during the 1930s, and the pattern of observation, reflection and recollection which gives this book its texture begins to emerge.

Malraux's ability to create images is exceptional, and his description of seeing Guernica in its unfinished state is especially vivid. Moved by the enormous scale of the canvas, Malraux inquires about the small preparatory drawings which Picasso had placed along the bottom of the painting. Picasso comments: I'd like them to move right up and put themselves in the canvas, climbing up all by themselves, like cockroaches!"

Reminiscences continue as Malraux and Madame Picasso come upon a room of Picasso's sculpture. These works - Bust of a Warrior, The Reapper, Woman with Leaves, and others - become a central theme in Picasso's Mask, as Malraux discusses their real theme in Picasso's Mask, as Malraux discusses their relationship to Western and non-Western art. What do these works mean - not as individual sculptures, but as the works of a genius placed in time? Some of the best art historical writing Malraux ever did is here, as his eye combs over the individual pieces and he describes the concepts created by the separate elements of each work. And Malraux is correct in emphasizing Picasso's sculpture; it is without precedent. Translating his paper collages first into welded metal sculptures and then into the cast bronzes made of assembled disparate parts, Picasso literally invented an alternate history of sculpture, divorced, from the tradition which had existed from the cavemen forward. Malraux notes that while other modern artists dreamed of sculptures that would rival in monumentality those of the Baroque period and of the 19th century, Picasso's small still-life objects changed history. They are, of course, part of a process of metamorphosis: "I often used pieces of newspaper in my collages," Picasso remarks to Malraux, "but not to make a newspaper."

As Picasso changed objects in making his art, so his manner of making art changed during his career. He defends his changes in style to Malraux: "I took some [hashish] once . . . Disgusting! For hours I was convinced that I would always paint the same way . . . Down with style! Does God have a style? He made the guitar, the Harlequin, the dachshund, the cat, the owl, the dove. Like me. The elephant and the whale, fine - but the elephant and the squirrel? A real hodgepodge!"

Thus the almost incredible procession of Picasso's works and their altering manner; the still-life compositions from 1912 to 1947, the variations on the theme of the Artist and his Model, his versions of works by Manet, Courbet, Delacroix and Velasquez, and his portraits. Picasso describes his persistent search: "I must absolutely find the Mask." The Mask is defined by Malraux as the transcendent style, what we sense in the Romanesque face in harmony with its god, the African sculpture endowed with a soul. The modern mask, Picasso's Mask, remains unknown.

To face the question of what is art Malraux returns to his earlier theory of the Museum Without Walls, first detailing his memory of discussing the idea with Picasso, and then recalling a visit to an exhibition devoted to this concept held in 1973 at the Fondation Maeght in Saint-Paul-de-Vence. "Cezanne's Louvre," says Picasso in showing Malraux his examples of "primitive" art, "probably wasn't very different from mine. What is very different are the works that aren't in the Louvre."

Malraux's thoughts in the Saint Paul exhibition range across the spectrum of the history of art. Our fascination comes in observing him seeing his idea of the Museum Without Walls now installed within the walls of a museum. The juxtapositions of cultures and the resurrection of art and artist - from Oceanic sculpture to Georges de la Tour as described in Malraux's writings - was made actual at the Foundation Maeght by joining the works themselves. Since the birth of modern art the museum has begun to enfold works it excluded before. The metamorphosis of historical items (created by people with no concept of art) into work of art - in our museums - is, as Malraux rightly observes, one of the great revolutions of the 20th century.

The other is modern art, and the weight of the book appears as Malraux links the two in his speech given at the opening of the exhibition, which is printed intact in Picasso's Mask. Modern art has become painting about painting, and as it has developed it has reflected a changing view of the history of art. A Rembrandt Self-Portrait is not seen for its nearness to the artist's face, but for what the art in it expresses. Look at Jackson Pollock's stylistic origins - EL Greco, Thomas Hart Benton American Indian art, Michelangelo and Picasso - a sourcebook unthinkable 100 years ago.

Modern art is in tandem with what has been enrolled as art in our modern world. "The Museum is gathering together works of successive or various civilizations at a time when styles are no longer considered interpretations of nature, but rather meanings of the world," writes Malraux. Chinese archaeological objects are shown at the National Gallery, Scythian gold at the Metropolitan.

Picasso's Mask is an expansion and modification of Malraux's earlier, influential Museum Without Walls, made considerably richer by his continual discussion of Picasso. There is much to agree - and to disagree - with here, accompanied by ideas that need probing. But with a grand sense of the scale of history, supported by an intimate reaction to works of art, Picasso's Mask confronts major issues of what art is.