What's more important than what? A man was killed, and then this show very nearly didn't come, which would have been a tragedy, but heck, tragedies happen. Art isn't food. It's more like a drug: you can live without it, except when it's got into your system, and then you think you can't. If Matisse's The Dance hadn't arrived at the Royal Academy you'd have had to go to St Petersburg to see it. Or perhaps you'd never have seen it. Whichever, it wouldn't have killed you.
The branch of Itsu where Alexander Litvinenko was irradiated is more or less opposite the Royal Academy, which brings the issue into nice proximity. And suppose the RA, instead of making frantic diplomatic efforts to get the Russians to lend us their pictures, had said: if that's your attitude, then stuff your art, and stuff our visitor numbers, just hand over the hit-man – and obviously lost the show as a result – would that have been a tragedy?
A cartoonist or a jokey contemporary painter could do a picture called Matisse-Polonium, in which the radiance of the master's colours is given a distinctly radioactive tinge. Moral: your artistic pleasure, on this occasion, is obtained by the soothing over of a crime. It's not a matter of art being irrelevantly dragged into politics. Like most of the world's goods, art rests on power, and there is sometimes a price to be paid for it, or to be refused.
From Russia: French and Russian Master Paintings 1870-1925 opens on Saturday. It has more than a hundred paintings, borrowed from Russia's four major public collections – the Pushkin Museum and State Tretyakov Gallery in Moscow, and the Hermitage and State Russian Museum in St Petersburg. Many of them have never seen in this country before. A good few are well worth seeing. And one or two are the sort of pictures that some people, in some moods, might say they valued more than life itself – or somebody else's life, anyway.
The basis of the show is that these four state galleries have inherited the exceptional private collections of two Russian textile merchants. At the start of the last century, Sergei Shchukin and Ivan Morozov paid keen attention to art in Paris and bought/commissioned works by Monet, Cezanne, Gauguin, Picasso, Matisse and others. Thus some classic pieces of early French modernism ended up in Russia, alongside their avant-garde Russian contemporaries, Chagall, Goncharova, Tatlin, Malevich.
In the exhibition, French and Russian works are interleaved, roughly chronologically, and you can try to follow it as a journey through one of the most dynamic periods of European painting. Ism begets ism. Realism, Impressionism, Post-Impressionism, Symbolism, Primitivism, Fauvism, Cubism (and its Russian variant Cubo-Futurism), and finally pure abstraction...
That doesn't really work, though. From Russia certainly comes to a strong conclusion. Kazimir Malevich's black-on-white trio, Black Circle, Black Cross and Black Square are a real punctuation point in painting's history. But the lead up is too random in its examples. You see how incredibly exciting the post-Impressionist liberation of colour was all round. You see how stylistically miscellaneous turn-of-the-century art was, before the isms got sorted and ordered. But a representative survey this is not.
Equally you can try and take the show as a collection of individual masterpieces. But, my goodness, that won't work either. I don't know who determined the selection, but what you have is a mixture of the terrible, the dull, the interesting, the good and the great. The quality of the painting, and especially the Russian painting, careers wildly.
All right, that's the kind of snotty, judgemental thing that critics are always saying. But these comparative judgements are not really a matter of cool and expert appraisal. They're not like ranking top dogs and prize marrows. They're not made. They seem to make themselves. You come across a work that is so strong that – when you're under its spell – anything less is knocked out of consideration.
I went round From Russia approximately twice, and the first time I didn't really look at the Matisses. That was partly a reaction against the pre-publicity, which has picked out The Dance as the cynosure of the exhibition, the one painting we all should be dying to see. And it was partly to do with half-believing the pre-publicity, having a feeling that Matisse probably was the star of the show, and saving him up, like the best bit of food on a plate, till the end.
I went round first in a rather disengaged way, trying to take it all in – confirming some prejudices, or revising them, and generally on the look-out for curious and unfamiliar things. And that generated a perfectly respectable set of responses. Like: what a fussy painter Gauguin is, always putting in too many bits and pieces that he doesn't know how to manage. And how crude Cézanne can be, even though we think of him as working with infinite care. The earlier of his two Mont Sainte-Victoires here is a shuddering, convulsive landscape. The terrain seems about to throw up. Why? Because its bulbous forms are outlined in cartoonist's shudder lines. But Manet – as so often, is amazingly sophisticated. In The Bar makes every element a perfectly timed surprise.
Or again: how eccentric and time-warped some of the Russians are. You'd never guess that Piotr Miturich's Portrait of Arthur Lourié was from 1915. It looks like a cool 1960s graphic. And Natalia Goncharova's Pillars of Salt is quite postmodern in its open jokiness. But what to say about all those "symbolist" images of people stuck stiffly in daft mime-artist poses – could they ever really have looked primitive and ritualistic? And as for Kandinsky's abstraction, it's just an enormous tizzy in paint.
The second time round, I stopped at the Matisses. And The Dance really is impressive, in the way it's made up of raw colours and crudely energetic elements – with a moment when the picture ought just to break down, yet is composed into something solid and serene. But this very big painting is essentially a public work, a wall painting, designed to be grand and stirring, but not utterly absorbing.
The Red Room, on the other hand, seemed about as rich and transfixing as a painting could be. The subject is frankly boring: an interior of tablecloth, wallpaper, fruit bowls, woman and window. But the flooding reds become a dense medium in which the details of the world are suspended, and Matisse's organisation, by which the disparate ingredients are all maintained at maximum intensity, and the way that the scene through the window goes off into quite a different register, ending top-left in a completely surprising pink house, which is still held within the picture's order – well, this is painting at full strength and complexity.
And, coming away from it, things looked different. Other works, which had previously appeared perfectly creditable, were now hardly worth looking at. And those that had seemed a bit stupid (the mime artists, the cubo-futurists) were now utterly pointless and unnecessary, not really paintings at all.
It felt embarrassing to return to a sweet picture like Tatlin's The Fish Seller and realise that, for all its nicely, lightly improvised rhythms, it was – compared to that Matisse – helplessly weak. Even Braque's The Castle La Roche-Guyon, a beautiful bit of luminous quartz cubism, could hardly survive next to it. So could one honestly wish not to have such a marvellous work on one's doorstep? Just for the sake of some international incident thing?
Well, what's art worth? It's a famous puzzler. People can be genuinely flummoxed by the thought of a Rembrandt and a baby in a burning building. Keep the competition strictly among artworks, and it's fine, of course. Get the Matisse out before the Braque, no question. But the superiority of one artwork to another doesn't make art itself superior to anything. And there are occasions when, however nice it would be to have it, it might be better just to say no.
From Russia: French and Russian Master Paintings 1870-1925, Royal Academy, London W1 (0870 848 8484; www.royalacademy.org.uk) Saturday to 18 April; to see more images from the show, go to independent.co.uk/art