On the face of it, Belgium, with its traditional bourgeois values, its solid creature comforts, its appetite for fine food, its historic city centres, is unlikely territory for the nurturing of revolutionary art. Events have proved otherwise, however, and the massive Magritte exhibition at the Royal Museums of Fine Art in Brussels is the second in a series of major Belgian Surrealist centenary commemorations.

The first was devoted to Paul Delvaux, who was born in 1897 and died only four years ago. While Delvaux insisted that he was not a Surrealist, no other label may be reasonably applied to his painting. How else describe a composition featuring a bowlerhatted city gent, moving away from a classical arch, immersed in his newspaper and wholly oblivious to the presence of two shapely females clad only in luxuriant headdresses of trailing ivy?

As a young man, Delvaux stated that he drew exactly what he saw, but with the passage of time he looked far beyond reality, and travelling by unfathomable channels of imagination, attained an enchanted world that remains as beguiling as it is inconsequential.

Rene Magritte is something else again, a sterner figure with a sterner message - or anti-message, depending on individual interpretation.

"My paintings," he announced, "are meant to be material signs of freedom of thought." In his case this meant total license to disassociate objects from the tyranny of conventional meaning and to fragment them at will; to transmute the common evidence of the senses - as in Discovery, where the skin of a female nude is mixed with grained wood - and to canvas his friends for arbitrary titles that bear no obvious relation to the subject.

"This is not a pipe."

"This is not an apple."

"This continues not to be a pipe."

Easy to accept when a moment's reflection prompts the realisation that the objects can be neither smoked nor eaten. But normal intelligence may well baulk at a handbag labelled Le Ciel, or a hammer masquerading as le desert.

The random association of words and images through which Magritte sought to challenge reality and change traditional thinking remain largely impenetrable. The Ligne de vie lecture that he delivered in 1938 goes some way towards illuminating his personal artistic canon, but there is an easier and at least superficially more attractive theory on offer from Lewis Carroll.

" `When I use a word,' Humpty Dumpty said, in a rather scornful tone, `it means just what I choose it to mean - neither more nor less.'

`The question is,' said Alice, `whether you can make words mean so many different things.'

`The question is,' Humpty Dumpty said, `which is to be master - that's all.' "

Approached in this spirit, there are no further problems with wayward nomenclature, and it becomes possible to appreciate the vast collection of this immensely sophisticated and enigmatic work without undue mind-bending. Magritte might not approve such a facile attitude, but surely freedom of thought remains freedom of thought no matter who is exercising it?

Even during the early days of dalliance with the Impressionists, and then with the Futurists and Cubists, Rene Magritte was never less than proficient. His art studies began at school when he was 12, but they had to have been severely disrupted by his mother's suicide two years later. When her body was recovered from the River Sambre after about a fortnight, the face was mercifully covered by her gown, and despite Magritte's persistent reticence about his private life, it is difficult not to find echoes of this tragedy in the headless torsos that appear frequently in his pictures, and especially in the shrouded heads in his two studies of The Lovers, painted in 1928.

His adoption of the predominantly literary doctrine of Surrealism launched by Andre Breton's Paris manifesto took place in 1925, with the development of a figurative language that made paradox the vehicle for potential conflict between the world and its visual representation.

Nocturne is one of the earliest examples of the new discipline. There is a painting set within a painting and a burning building at the centre; there is a stage curtain behind, and a red bird flies forward from the inner frame towards the musical bilboquet that occupies the foreground. Stage curtains and musical bilboquets are present again in the beautiful and evocative partnership of gouache and collage that form the Jockey Perdu of 1926, but the curtains vanish and the bilboquets lose their decoration in the version of the same subject painted 30 years later. In the first picture, the bilboquets sprout dark, bare branches to create a wintry landscape; in the second they are generously clad in foliage, while in each case the same horseman whips his mount onward in evident panic - all of this confirming Magritte's statement that he derived thousands of paintings from only "a hundred or so" images. Some are actual duplicates, and to this day dispute continues about the dating of the two virtually identical studies of La saveur des Larmes.

Along with bowler hats, sleigh bells and pipes, the bilboquet was quickly established as a hallmark of Magritte's style. Technically a toy or weighted, self-balancing figure, in his expert hands it resembles a chessman of the lower order, or possibly an outsize black-pepper grinder; and it is constantly used as embellishment. Throughout his career, Magritte painted firmly, clearly and smoothly, and despite the frequent element of eroticism, with an extraordinary lack of sensuousness. Even the once-notorious impression of rape, in which a woman's face is convincingly replaced by the sexual attributes of breasts, navel and pubic hair, stirs no emotion. No more do the coffins substituted for the recumbent Madame Recamier of David's inspiration, and the people of Manet's Balcony.

None of them compares with the power of The Domain of Arnheim, which reflected the painter's admiration for Edgar Allen Poe, or with the blazing tuba of The Discovery of Fire, though any suggestion that Magritte is indifferent to the human form, or its potential metamorphosis, is conclusively denied by the familiar bowler-hatted man, who achieves delightful apotheosis in Golconde, with dozens of tiny look-alikes descending on, or maybe ascending from, a suburban roofscape.

Metamorphosis assumes many forms, and Magritte's preoccupation with the process reaches its most spectacular expression in the "petrification" series of the 1950s. He displays stunning virtuosity in the painting of glazed and pitted stone, transforming men, books, animals, fabric and whatever else took his fancy into this unaccustomed medium. The Castle in the Pyrenees is probably the best-known of these remarkable compositions, but to my mind, there is more to wonder at and revere in Memory of a Journey, where stone man and stone lion dominate stone floorboards, with, behind them, a stone table and its stone furnishings, stone walls and a stone picture.

Between 1949 and 1962 Magritte made no less than 16 versions of The Dominion of Light. The three oil paintings on show at Brussels date from 1952, 1954 and 1962, and although all of them have the same street light burning outside darkened houses with lamplit windows, the disposition of buildings and accompanying trees is different in each.

The incongruous daylight sky above the night-time houses varies in cloud arrangement from one picture to the next, but except for a full moon against a background of succulent leaves, this masterly amalgamation of night and day is presciently diminished in his final work, painted three months before his death in 1967, and named sadly and almost certainly with foreknowledge, The Blank Page.

In addition to more than 300 paintings and gouaches, the Brussels show covers substantial commercial work Magritte practised grudgingly but assiduously, to keep body and soul together. The influence of his posters is still discernible today in advertising, particularly on television, while many fashionable operatic productions have their roots in the painter's adventures in photography and cinema.