Private view

A commonplace of art history is that, in times of anxiety or extreme formal experimentation, modern artists - from Degas to Seurat, Renoir to Matisse - looked for inspiration to Ingres, the great upholder of the French classical tradition. So when Matisse was flailing in the wild colours and brushy incoherence of his Fauve period, he turned to Ingres for an ordering principle. The results were the Matisse decorations of 1909-10. Renoir, resisting dissolution into the impressionist soup (derived from Ingres's great rival, Delacroix), looked to Ingres's odalisques when painting his late, buxom bathers. With Degas and Seurat it was the same: Ingres helped impose structure and line on to pictures flirting with deliquescence.

There is one modern artist, however, who looked at Ingres more than any other, and a compact exhibition in Paris shows that what he found in the cantankerous old Cerberus of the classical tradition was not so much discipline and order as the habit of heresy.

"Picasso Ingres" at the Musee Picasso is one of the most electrifying shows I have ever seen. Despite its size, it is as important as the recent international blockbuster "Matisse Picasso." And like that exhibition, which went some way to lifting the fog of cliches insulating those two artists from fresh reception, "Picasso Ingres" alerts us to the dangers of art historical shorthand - of thinking about individual artists through the prism of generalisation.

When Odilon Redon was pressed about the strange quality of some of his pictures, he protested, "but it is Ingres who has made monsters!" Picasso - art's most notorious monster-maker - would have agreed. When looking at Ingres's La Grande Odalisque in the Louvre, it is customary to remark on the anatomical impossibility of her endless spine. Similarly, in front of the supplicating figure of Thetis in Ingres's Jupiter and Thetis, one French endocrinologist claimed that her erotically swollen neck was evidence of a malfunctioning thyroid gland. We may assume the explanation for these anatomical distortions lies in Ingres's wish to subordinate all the elements of his pictures, even the sovereign human body, to the dictates of a classical conception of grace and order. But that is to glide too easily over what surely arrested Picasso: the morbid voluptuousness of Ingres's bodies, and the simmering surrealism of his combination of photographic fidelity to appearances and physically impossible phenomena.

Many of Ingres's distortions are co-opted throughout Picasso's oeuvre. Thetis's rubbery gullet is pinched for one of the despairing victims in Guernica. The starfish fingers supporting the head of Ingres's Madame Moitessier, Seated reappears in Picasso's 1937 Portrait of Dora Maar. And this exhibition contains dozens of similar examples of Picasso's "lightning raids."

Picasso and Ingres were the greatest draughtsmen of their respective centuries, so it makes sense that the Spaniard should have looked for inspiration to the Frenchman. Both artists' careers contained a huge variety of styles, adapted to subject matter that was almost perversely eclectic. But their intense focus on line opens on to some of the most compelling qualities of their work.

The first is invention: line goes (if you are good enough) where you want it to go, and the linear mastery of both these artists gave them the confidence to take sense-altering liberties. The second is lucidity: even the most complex of their compositions - one thinks of Ingres's Odalisque with Slave or Picasso's comically pornographic etchings on the Ingres-inspired theme of Raphael and the Fornarina - are made legible and clear through linear assurance. The third is flatness: concentrating on line over modelling emphasises surface tension and contracts three-dimensional space, which helps explain why a picture like Ingres's Turkish Bath can seem like a prototype of Picasso's cubism.

Ingres's own tensely rippling flat surfaces took their cue from the linear purity of Greek vase painting, as well as the Greek-inspired outline illustrations of John Flaxman, which were all the rage in Napoleonic France. Ingres's love of the Greeks sprang from his desire to evoke a sort of legendary remoteness. This yearning for unsullied origins, for an art historical tabula rasa, is a key to the exhibition, since it is the same feeling that motivated Picasso in his Rose period when he painted masterpieces such as Boy leading a Horse, Woman with a Fan, and Boy with a Pipe, which sold recently to an anonymous collector for a record $104m.

Picasso looked closely at Ingres again in his own neoclassical period, when he went to Italy and began turning out portrait drawings deliberately resembling the portrait drawings Ingres made during his own stint in Rome. Baudelaire rightly observed that Ingres imposed a preconceived idea upon his sitters. It was this idea, as much as the draughtsmanship, which has travelled.

After looking at John Flaxman's outline drawings, Ingres's teacher, Jacques-Louis David, said, "This work will cause pictures to be painted." One could say the same, with confidence, about this wonderful, provocative exhibition.