SIGNPOSTS TO THE 20TH CENTURY

It's a year since Cezanne set the standard for the blockbuster exhibition. Now Piet Mondrian could be about to match it in a huge retrospectiv e at the Tate Gallery.

The Tate Gallery's exhibition of paintings by Piet Mondrian doesn't open for another three weeks, but already one senses the mixture of excitement and curiosity that accompanies the retrospective of a great artist. Curiosity is relevant. Mondrian was a most peculiar man, even though his painting was so lucid and his way of life was simplicity itself. Ambition explains some of his peculiarity. Modest in worldly ways, he does seem to have believed that all art - the whole history of the world's art - had merely been a preparation for his personal vision. This humble, retiring man had an ego as big as a house.

Mondrian could not have been so original an artist without that egotism. There's something wonderfully unfaltering in his pursuit of abstract beauty. He was all the more radical for being sure of himself, never looking sideways, his eyes fixed on some invisible future. Furthermore, his good fortune was to have lived in times when idealism and the notion of progress came naturally to innovative artists. Mondrian is a pure example of a man with such attitudes. His idealism was hardly affected by two world wars and a life spent largely in exile; and perhaps foreign cities helped him to concentrate on himself, just because he felt so little attached to any of their customs and traditions (and, one suspects, their inhabitants).

Amsterdam, Paris, London and New York were his places of work, the significant capitals of western Modernism. Like the geographical itinerary of his career, Mondrian's painting styles also form a sort of arch over the development of 20th-century art. When he began painting in the 1890s his landscapes and still-lifes were rather stolid pieces of Dutch naturalism. Then he entered a symbolist period around 1908, at which point we may say that he joined the mainstream of modern art. By late 1911 he had settled in Paris and developed his own version of Cubism. Then he started to make some of the very first abstract paintings and for decades his abstraction grew in authority as he sought the most absolute statements. His final home in New York allowed him to have some part in the emergent Abstract Expressionism.

He wasn't really close to the abstract expressionists personally, though in 1943 this septuagenarian veteran of European art was perspicacious enough to see the merits of the young Jackson Pollock. I've seen just one Mondrian painting that connects him with the moody, inward lyricism of Pollock's generation. Perhaps it's wrong to call it a painting, since there's no pigment on the canvas. It's just some exploratory charcoal lines. Mondrian was evidently judging where his characteristic rectangles might be finally placed. The lines do not, however, look like a scheme. Miraculously, they resound with a character that one can only call spiritual. Had the picture been completed, flat oil paint and simple colours would no doubt have given it the crispness and finality we so often find in Mondrian's mature art. This glimpse into his working methods was (for me) a useful reminder that Mondrian was a brooding and religious man.

The nature of his personal religion is hard to discover, for a number of reasons. First, it was private. Second, Mondrian's few writings about his beliefs are terribly obscure, whether they concern art, religion or any other topic. Third, he was a theosophist, and the tendency has been to regard his theosophy as mere crankiness. And who wants to study Madame Blavatsky rather than look at a painting? But a more tolerant view of the modern and artificial religion might find that it suited Mondrian's artistic purposes rather well, for it insisted on the destruction of the old orders and the creation of new and higher forms. At the least, Theosophy released Mondrian from the somewhat grim Calvinism of his family background, and it cannot be a coincidence that he took it up in 1908-9, just at the time when his painting took a new and exploratory path.

That path was related to movements in Paris and elsewhere. In paintings of the time we see fauvist colour, divisionist brushstrokes and an interest in the mysticism of Jan Toorop and Ferdinand Hodler, still regarded as advanced artists in the Holland of the day. The Red Tree, Mill in Sunlight and a series of paintings of dunes are of this type. We see that Mondrian could do accomplished exercises in several new art forms. Only one thing was missing: the undraped human figure. Like another ex-Calvinist, his fellow countryman Van Gogh, Mondrian could not manage to paint the nude. His most overtly theosophical work, the three-part Evolution of 1910-11, is a weird picture in which three naked women are said to symbolise the union of matter and spirit, or the cosmic arrangements of the universe beyond time. One hesitates to call Evolution a bad painting, but it probably satisfied Mondrian more than anyone else who has had to consider its message.

Theosophy was best when not depicted. It lay submerged in Mondrian's mind and character while his painting engaged with more concrete problems. Mondrian never felt that he was a provincial, but he had tasted French art and he knew that Paris was the centre of innovation. So he went there and stayed for some two and a half years between 1912 and 1914, living in a spartan studio near the Gare de Montparnasse. The Parisian avant- garde was ruled by Picasso and Braque, as Mondrian must have known, yet there is no evidence that he met either of them and he cannot have had a thorough knowledge of their work. Cubist paintings were studied by a coterie to which Mondrian did not belong. In the two pictures called Still Life with Ginger Jar, then in the oval compositions of 1913-14 and the Facades - which he derived from Parisian architecture - there are striking similarities with contemporary Cubism, but Mondrian probably worked these paintings out on his own.

Picasso's and Braque's Cubism was based on still-life. They avoided nature. Mondrian by contrast organised for himself a style parallel to Cubism by looking at trees, landscape and architecture. We know that the buildings in the Facades paintings were close at hand, but surely the trees were distant, growing in Mondrian's mind rather than in soil. I think his first Paris years confirmed Mondrian as a visionary who removed himself from common concerns. Some photographs of the painter are interesting. In the earliest of them we see a young man who looks charming and might also be a bit of a sensualist. Then there are photos of a bearded theosophical Mondrian in meditational poses. In Paris he shaved off his beard, began to wear rimless glasses and wore a severe though slightly shabby suit. Mondrian lived on soup, so was thin. He looked like a provincial bourgeois museum official down on his luck, and who could have known what was in his unusual imagination?

For Mondrian was not a natural communicator and his painting must have seemed nonsensical to all but a few modern spirits. He met some people of this sort when the First World War broke out and he returned to neutral Holland. Thus Mondrian became associated with the loose grouping known as De Stijl, founded and promoted by Theo van Doesburg. De Stijl was interested in architecture and design as well as painting. It had mystical and utopian tendencies, embraced abstract art and was self-consciously modern. These characteristics suited Mondrian, but he was not a joiner by nature and wasted a lot of time quibbling over manifestos when he could have been at his easel. De Stijl is an important movement and recently it has attracted dedicated historians. Some of us, though, weary of their researches, and for a very simple reason. Most members of the group were dull, while Mondrian was so good.

He was both a natural painter and a dedicated aesthete. The two qualities do not always go together, but we may see their conjunction in the - all too few - paintings derived from Dutch landscape in the years before 1919, when Mondrian returned to Paris. Their questioning and exploratory character, exquisite mark-making facility, gentleness and high intelligence combined to make a standard for abstract art: a standard that no other pioneer of abstraction could approach. Intriguingly, some pictures like Pier and Ocean or are not totally abstract. They are between nature and artifice, respond both to the Dutch coast and some lofty ideal of modern form. Holland, with its dykes, pollarded willows, neat allotments, straight paths and canals, is a country in which nature has been carefully disciplined by mankind, and surely this aspect of his native land is embedded in Mondrian's painting.

Mondrian was 47 when he went back to Paris after the First World War. He remained in France until the late 1930s, and from about 1921 was in command of his greatest period. Simplicity, economy and rigour seem to be the watchwords of his new style, but that's at first sight of the pictures. When one considers them more thoughtfully (and not in reproduction) it becomes apparent that they are heartfelt and also daring. Everything in them is perfect and they form perfect wholes, yet by the standards of normal pictorial behaviour they are illogical. It's as though the conventions of symmetry had been reinvented by a person whose home was in the ether, not on earth. The paintings never resemble each other and their artist never copied them. It might be added that no one has ever managed to fake one of these pictures, or not convincingly, though there have been a number of attempts by people who think that they are straightforward. In truth Mondrian's best paintings are immensely complicated. Relations of area are judged to a millionaire, but within those relations some lavish form of creation has taken place.

The wonder is that Mondrian made such inventive art from anonymous paint application and a palette restricted to the three primary colours plus black, white and grey. I expect that there will be many wonders in the exhibition. One keeps discovering things in Mondrian. Someone at the Tate has had the excellent idea of asking Bridget Riley to choose and install the show. Her fine eye will surely give us a different view of a complicated painter. The exhibition will not, however, be complete. It doesn't cover Mondrian's London or New York years and practically all the exhibits come from a single source, the Gemeentemuseum in the Hague, closed at present for renovation.

Mondrian's last years in London and New York are a coda to his career, though his few American paintings announce a complicated new departure in his style. In 1938, for the second time in his life, he was obliged to leave Paris by the threat of war. This time there was no question of returning to the Netherlands. Instead he went to London. His English sojourn was arranged by a small group of admirers, the abstract artists Ben Nicholson, Barbara Hepworth and Naum Gabo prominent among them. They found Mondrian a studio in Hampstead (at 60 Park Hill Road, really more in Chalk Farm than Hampstead village). As usual, he stripped his new dwelling place nearly bare, then painted his few pieces of furniture white, placed cards in the primary colours around the walls, and began to work. Chalk Farm life was an interlude, but there's a salute to London in the majestic painting Trafalgar Square.

That picture is now in the New York Museum of Modern Art, for Mondrian took all his canvases with him when he sailed for America in 1940. Far distant from the war, and in a city that was filled with artistic refugees from Europe, Mondrian appears to have been a contented if not exactly a happy man. In his frail old age he took part in artistic activities and began to renew his painting. The stimulus was also in the physical structure of New York itself, the most modern city Mondrian had ever known. One of the odd things about this reclusive and reserved painter is that he loved jazz. He had studied the new music since the early 1920s. In New York he was glad that jazz was all around him, and his last completed paintings are called Broadway Boogie-Woogie and Victory Boogie-Woogie.

These pictures have often troubled Mondrian's admirers. They are filled with very small squares, and Victory Boogie-Woogie is lozenge-shaped, a square set on its point. Perhaps they respond to the nature of New York architecture, or conceivably to musical rhythms. In either case, they are tightly and regularly constructed in a manner that contrasts with Mondrian's Parisian art. One might say that he was close to the architectural side of his painting in New York, while in Paris he had been more metaphysical. So the people who most appreciate the ethereal side of Mondrian are not always convinced by the final American paintings. They look as if it might be possible to copy them. We don't have this feeling when looking at Mondrian's most consummate art.

! 'Mondrian: Nature to Abstraction': Tate Gallery, SW1 (0171 887 8000), 26 Jul to 30 Nov, sponsored by AT&T. Tickets are pounds 5 (pounds 3 concs), and may be reserved through First Call (0171 420 0000), subject to booking fee. Bridget Riley will talk about the show on 25 Jul at 3.30pm (pounds 12).

Piet subjects: above, 'Evening on the Gein' , thought to have been painted in about 1906, when Mondrian was emerging from his period of Dutch naturalism; below, a development of style in 'Oval Composition' (1913)