Matisse: An Old Master who loved to learn new tricks

It would be almost impossible to produce a bad exhibition of Matisse.

You enter a room of his works and your eyes are transformed. It's the colour of course, those reds and deep greens, always fresh and balanced. And then there's the rhythm, the curved lines of flesh and flower and the soft straight lines of wall and window. But most of all it is something always uplifting in the way that he seeks and communicates the harmony of life.

A new exhibition at the Centre Pompidou in Paris goes further than any I've seen in explaining what it was that made him tick as an artist. It's built around the theme of "Pairs and Series". During most of his creative career Matisse painted pairs, and later whole series, taking a view or a composite image and working it this way or that in tandem with each other. It started early on with two studies of "Still Lives and Oranges" from 1899 when he was just beginning to develop his own style and sensing the influence of other modern painters. The subject is identical, but in one he explores space and colour, in the other light and texture.

The differences become even more marked as he continues and experiments with more varied and radical styles. Three views of Pont Saint-Michel from around 1900 has him painting one in the impressionist style of Monet, another in the manner of the Fauves and a third in which the picture is pared down into blocks of colour giving a primitive effect. In a glorious set of three portraits of his daughter, he paints two in a naturalistic style with a light brush and pastel colours. Then, with the arrival of the Spanish artist, Juan Gris, in Collioure where he was living, he dramatically changes focus to portray the girl with a cubist face of fierce vertical lines. In the most striking pair of all, he views Notre Dame in 1914 on the one side with a watercolour-like sense of liquid effect, and the other as a virtual abstract of geometric lines and form in a canvas suffused in blue.

This is much more than an exercise in trying out influences, although Matisse, like Picasso, was endlessly concerned with the art going on at the time and what he could learn from it. Rather it is the constant effort of an artist to find the way into his innermost feeling for the world immediately round him. A model described how he would start two works off. One he would finish relatively quickly over a few days, the other he would work on for weeks and sometimes months, always refining, altering and sometimes completely altering until he'd got it right.

"The reaction of each stage," Matisse, always supremely articulate about his work, wrote later "is as important as the subject. It is the basis of my interpretation that I continually react until my work comes into harmony with my feeling. Like someone writing a sentence, rewrites it, makes new discoveries. At each stage I reach a balance, a conclusion. At the next sitting, if I find a weakness in the whole, I find my way back into the picture by means of weakness. I re-enter through the breach and reconceive the whole."

His constant concern is with space and how you can create it out of objects or simple colours. His continuous ambition was to keep trying different ways to achieve balance. There are moments when he puts himself into the picture, usually discreetly in shadow or part, there are paintings in which he puts in paintings of his painting to reflect the artist's relationship with the life he is recording. But although his is often an intellectual approach it is never a distant one from the viewer because it is never distant from himself.

Together with pairs, he also produced series built around an initial image, most gloriously in the sets of drawings of the female figure and mundane objects which he gathered together in his Themes and Variations in 1940-1. An enlarged photograph of his studio at the time reveals them pinned to his wall in serried ranks from floor to ceiling. The exhibition also has the photos he took of the various stages of his main paintings as he worked on them. They helped pin down what he had done, where he had changed the details or form, to help him on to the next stage before he considered the work complete. On his instructions the photos were blown up and displayed with the final canvases at his post-war Galerie Maeght exhibition in 1945 as a demonstration of his method to anyone who came. What is extraordinary is the number of times he repaints virtually the whole canvas before working it again, to end up with compositions not totally different but worked through until he felt he'd got precisely the image he'd sought.

The exhibition climaxes with the two masterpieces that resulted, The Romanian Blouse and The Dream from 1940. To stand before them is to have your heart lifted. It ends, as it should, with his final full-size cut-outs of the blue nudes in which all his craft was used to inscribe humanity in its purest shape. An exhibition of total wonder and pleasure.

Paris has lost out in recent years to London with the big travelling shows. But no one does their own art better. As with Matisse, so with that other great experimenter and humanist, Degas. Degas et le Nu at the Musée d'Orsay in many ways parallels the Royal Academy's recent show of his dancers. As with ballet, so Degas pursued the theme of nudes throughout his life, pushing the contortions into which he could fit the female form to its limits, experimenting with techniques from pastel to tracing paper to get just the right effect. Fascinatingly, he started with contortion in his first pursuit of history painting and ended with his late series of women in the bathtub and combing their hair. In between there is a quite touching, and to me quite new, series of monotype small pictures of women in the brothel at their ablutions and waiting for customers. The technique is extraordinary. Yes, there is something a little disturbing about Degas' eye when it comes to women but there's nothing odd about his pursuit of human form.

Matisse: Paires et Series, Centre Pompidou, Paris (centre pompidou.fr) to 18 June; Degas et le Nu, Musée d'Orsay, Paris (musee-orsay.fr) to 1 July