After Impressionism review – radical ideas and ecstatic sex from the edge of a new universe

‘Make it strange” is one of the slogans of modernism. The National Gallery’s show After Impressionism makes modernist art itself strange, by seeing it from the past – the Victorian salons where this revolution in the arts actually started. It is a flawed show but one I found hard to leave. European art in the 1880s and 1890s hurtles towards the “modern” before your eyes, yet also burrows away into recesses of nostalgia and pastoral – and you lose yourself, as modernism wants you to.

You can cut a line through the exhibition and follow the high road of the new, ignoring all those odd byways. Simply rush from Paul Cézanne’s Mont Sainte-Victoire with its hypnotic field of broken, tentative, obsessive dapplings held together by an iron intellect, straight to Pablo Picasso’s 1910 portrait of Wilhelm Uhde. This writer and collector is the last man, the last bourgeois individual, in Picasso’s revolutionary portrait. His cartoonish features, pinched and prissy over a stiff wing collar, are disintegrating into a crystal cavern of invisible structures made suddenly visible. This is the maze of “cubism”, that takes its start from Cézanne’s analysis of vision. This is where, by 1910, the most radical art stood – on the edge of a quantum universe.

Cézanne initiated that. The biggest shock of the show is how much more serious he is than its other two supposed heroes, Van Gogh and Gauguin. Yes, that’s right – better than Van Gogh. That is the clear conclusion of a display of five works by each, facing each other. Vincent’s paintings are touching, intimate, yet traditional compared with Cézanne’s dismantling of art and nature. Gauguin meanwhile is brittle and strident, his art always trying too hard to be “mysterieux”.

Picasso is the gifted pupil Cézanne never met. You can see that by darting to the end to see Picasso’s Woman with Pears, a portrait of his lover Fernande Olivier done in 1909. Is it a portrait? Fernande’s head is massive and industrial. Like a taut spiralling girder for a modernist monument, her neck tendon shoots up in a curve of electrifying torsion. Her eyes are diamond studs in a face of jarring planes, her hair a heap of black croissants. Yet beside this immense mask of the new age are perfectly recognisable, simplified pears on a table. They are Cézanne pears. You can go back to room two and check, by comparing them with the fruits in Cézanne’s Sugar Bowl, Pears and Tablecloth.

This is the 50th anniversary of Picasso’s death. Modernism, the movement that sought to remake everything in art from a new, primal beginning, belongs to history now, but it does not get old. That is because it takes apart centuries of tradition in the name of a more basic truth. All the artists here are looking for truth, even if they don’t all find it.

Why does Degas make it into this show while other impressionists get left behind? Sex. Degas pervs with the best

They saw this deeper human reality as primal. “Primitif”, to be exact. Modernism was born in Europe’s age of empire. From Tahiti in 1892, Gauguin sends the poet Stéphane Mallarmé a wooden carving named after Mallarmé’s poem The Afternoon of a Faun. His extraordinary object merges classical myth with racial stereotypes to portray a goat-legged Polynesian man desiring a Tahitian nymph. You can’t fault Gauguin’s clarity. The sensual Arcadia that Mallarmé conjures is a real place in the Pacific, says Gauguin.

Even as Europe conquered or exploited much of the world in the 1800s, the “primitive” art that came streaming on to its markets exacted an aesthetic revenge. Artists preferred it to their own “civilised” conventions. When the Belgian artist James Ensor depicted a variety of non-European masks in Astonishment of the Mask Wouse in 1889, Belgium was running the most brutal of colonial businesses in the Congo, working thousands of people to death. André Derain owned a Fang mask from central Africa that directly influenced him and his friends including Matisse and Picasso. Derain’s 1906 painting The Dance portrays fantasy dancers, one with a painted body, another with a shadowed, mask-like face, another in an ancient Greek dress, in a golden paradise where they gyrate with unrepressed abandon.

If the first moderns sought to be “savage”, they also loved sex. This was the age of Sigmund Freud. Matisse’s Dance – no, not that one: who knows when any of us will see his masterpiece in the Hermitage again? – but a carved wooden relief from 1907, releases raw, ecstatic sexuality in its wild naked romp. And this is just one of the exhibition’s nude explosions. In Berlin they were fixated on flesh – tottering, quivering masses of it. Lovis Corinth’s Nana, Female Nude looks as if she’s been left over from the recent Lucian Freud show in this same space. Corinth’s Perseus and Andromeda, in which a knight in armour unveils a female nude, may not seem very modern at all: but nor does Wagner until you hear him.

Why does Degas make it into this show while other impressionists get left behind? Sex. Degas pervs with the best, or the worst. Facing Gauguin’s pensive isolated tropical nude Nevermore are his painting of a woman lost in red ecstasy as her long hair is combed, and pastel of a naked model curled up reading.

It’s a shame Pissarro doesn’t make the cut when he worked so closely with Cézanne and later helped pioneer the divisionist or pointillist style. It is equally baffling that Seurat’s compelling essays in this perceptual art get less space than Paul Signac’s painting-by-numbers attempts. And Munch is shoved into the Berlin section, oddly, which doesn’t stop The Death Bed harrowing your heart.

I could go on, and get cross, but it would miss the point. For some of the unevenness is in the period itself. What this show reveals is that modernism was an end, as much as a beginning. Five hundred years of European pictorial art – the very tradition the National Gallery displays – were breaking and decaying, and what was born in their stead was difficult, elusive, as daunting and inescapable as Picasso.