A portrait of the artist as an old man

Butterflies' wings. Tinfoil. Plaster. Leaves. Cement. Concrete. Dried flowers. Felt-tip pens. Children's crayons. Bits of broken bottles... These are just a few of the many materials from which the Paris-based painter Jean Dubuffet (1901-1985), creator of Art Brut ("raw art"), made his art during the second half of his long life - he came fairly late to such extravagant gestures of non-conformity.

Butterflies' wings. Tinfoil. Plaster. Leaves. Cement. Concrete. Dried flowers. Felt-tip pens. Children's crayons. Bits of broken bottles... These are just a few of the many materials from which the Paris-based painter Jean Dubuffet (1901-1985), creator of Art Brut ("raw art"), made his art during the second half of his long life - he came fairly late to such extravagant gestures of non-conformity.

Dubuffet was born in Le Havre into a prosperous family of wine-dealers, and for the first four decades of his life he was a dabbler in painting of a fairly conventional kind. He studied at the Académie Julian in Paris, though not for long. His earliest canvases, most of which were later destroyed because he came to regard them with some contempt as part of his "pre-history", show the influences of Suzanne Valadon, Cézanne, Dufy, Derain and others.

Later, taking his cue from the avant-garde spirit of the times, he became excited about Futurism and Dadaism. But the painting was an on-and-off kind of pursuit during those early years.

Then, in 1942, having found a reliable manager for the family wine business, he got stuck into his true vocation of mark-making. Even then he didn't take it too seriously, though. After all, wasn't he too old to make a career of it? Perhaps it would prove to be little more than a pleasurable pastime. Some demon within him thought otherwise.

Dubuffet was not only an extraordinarily versatile practitioner throughout the 40-odd years of his working life as a painter but also, a Frenchman to the core, he was a great theorist and systematiser with strong views about culture and beauty. Beauty, he once wrote, was a meagre and unintelligent invention. "It is distressing to think about those people who are denied beauty because their noses are crooked or because they are too fat or too old..." Art for him had nothing whatsoever to do with the pursuit of any ideal of beauty.

To see this theory in action, look at any of the human beings represented in this exhibition of his later paintings and works on paper. In a Dubuffet painting, human beings tend to be small, abdominally bloated, skewed of nose, and with hydrocephalic heads. They float around the flattened picture plane in bubbles looking laughably bedazzled or grotesquely, pathetically clownish.

But it was not beauty alone which Dubuffet's art lived to disparage. It was cultural tradition in general, which could include anything from notions of perspective to artists' materials. And who was fit to be a painter anyway? Not necessarily those who had been traditionally trained in painting. The source of art's true vitality, according to Dubuffet, was to be found at the margins of polite society, among the eccentric, the insane and the untutored, or in the children's playground. Hence the Outsider Art - Art Brut - which Dubuffet not only practised, but also collected with a passion from 1949 onwards.

The exhibition at the Waddington Galleries shows the fruits of the last 10 years of his extraordinarily fertile career as an artist from 1974 until his death. Dubuffet was forever changing his matter, his manner and his materials. (In his final decade, he was using acrylics on canvas-backed paper, crayons and oils.) Before his death he helped to organise an exhaustive, multi-volume catalogue of his entire artistic output. Each volume would represent a single phase of his work. The entire series runs to no fewer than 39 volumes.

By 1974, Dubuffet was coming to the end of a project which had occupied him for a decade - the Hourloupe cycle of paintings, drawings and monumental sculptures. This cycle is characterised by the use of artworks built up from jigsaw-like components, many fiercely crosshatched, and by the use of a very limited palette, notably red, white and blue. It was a decade which had seen him experiment with sculpture in polystyrene, a material that looks quite robust and monumental, but that in fact is very easy to work and almost as light as air.

The largest single work in this series is a cuboid room called Cabinet Logologique which he created for himself at his studio at Périgny-sur-Yerres in the Marne valley as part of a weird sculptural environment called "La Closerie de Falballa." The walls, floor and ceiling of this strange underworld grotto, which you enter by a pair of doors called "Paladin" and "Paladine", are entirely decorated in Hourloupesque fashion. Here, Dubuffet would lie on his bad back for hours on end, smoking and meditating upon the significance of his own creation. "The Cabinet Logologique," he once ruminated through a thick fug of smoke, "is... a chamber of philosophical exercise whose destiny it is to familiarise us with the illusory character of what is generally regarded as reality, and to make us aware of the multiplicity of possible realities..." Quite.

In 1974, he began to create various series of new works on paper and canvas, looser, freer and more frenzied than anything which had gone before, using school-children's crayons and felt-tip pens. One group, a series of small works on paper seething with graffiti-like figures, was called the Crayonnages, for example. A series called Non-Lieux abandoned the figure altogether and consisted of a maddened frenzy of lines, zipping from side to side and round and round like a swarm of gnats scribbling on the air on a summer's day. As so often in this late work, the lines are violently infantile, the colours primary. The older he became, the faster and more furiously he seemed to work.

By October 1975, he was creating a large series called "Theatres of Memory". The best - and the largest - work in this show, Site Agreste, dated 22 May 1977, belongs to that ongoing series. Site Agreste is essentially an assemblage from six smaller paintings, which also incorporates elements of collage. The whole is a clamorous mass of calligraphic signs, humanoid forms and primitive representations of trees, rivers, roads. Dubuffet was seeking in this period some means of displaying "multiple memories of places and people, that simultaneously crowd into our memory in every direction".

How important was Dubuffet? The question remains unanswered. He had many enthusiastic supporters, including some of the greatest French writers and critics of the 20th century - Jean Paulham, Paul Eluard, Francis Ponge among them. He painted in series, and the impact of individual series is often a collective one. He was not a man to labour over a single canvas, and the consequence of this feverish relish of spontaneity and series-building is that individual works are just as likely to be mediocre as brilliantly arresting. He is not a man whose work consists of a handful of masterpieces, though there are many great single works, several of which are on view in the Centre Pompidou.

He was, however, both influential and prophetic. His love of junk and random materials prefigures Pop Art. His gestural frenzy aligns him with some of his famous American contemporaries. And yet there is something faux-naïf and even self-absorbed about him, too. This prosperous, cultivated man was really playing at being an outsider.

Jean Dubuffet: Late Paintings is at Waddington Galleries, Cork Street, W1, until 30 Sept (020-7851 2200)