Monet, Monet, Monet

Landscape painting, William Hazlitt said, is "the obvious resource of misanthropy". For the viewer as well as the artist, it's a chance to get away from them all. But if you love the scapes of Claude Monet, his views of the Giverny garden, say, his wide stretches of pond water, then you'd better love your fellow humans too. You're not likely to escape them that way. Quite the contrary.

Nine years ago the Royal Academy staged "Monet in the 90s", and at the height of that exhibition the main thing it offered the visitor was a chance to study crowd-movements in close quarters. Now we have "Monet in the 20th Century". I don't think the artist's popularity has waned much in the interval, though perhaps the RA has got more adept in crowd- management. At any rate, it's curious to consider that in the pictures themselves - if you can catch a glimpse of them - you'll not see a living soul.

Now I must not gloat. I have seen this show. I saw it on Monday. And for quite a lot of the time - give or take the odd guard, fellow critic, curator and an easily avoidable TV crew - I was more or less alone in it. Of course, I'm very glad of this. But it does mean that the show I saw, and the one you may see, are likely to be significantly different. The exhibition opens tomorrow. If a rapid response hadn't been requested, I'd have gone back again in public hours, to experience real viewing conditions - and not just to gauge the practical problems. Seeing and the self and solitude are pretty central matters to Monet's late art.

But the first wonder is how long he lasted. Monet, arch-Impressionist - it was his Impression: Sunrise of 1872 that first inspired the name - lived on through Symbolism, Fauvism, Cubism, Futurism, Dadaism even. He died in 1926, aged 86. A third of his painting life fell in our century. Whether, by the end, his work can still be called Impressionist, whether it should really be called visionary or indeed abstract, are not entirely stupid or anachronistic questions. Contemporary critics asked them, and anyone may well wonder. Incidentally, it's not (as people used to think) just a matter of Monet having bad eye trouble.

Fast forward the video: that's usually good advice for any blockbuster, and it is here. Don't waste your feet or eyes, your barging or head-dodging skills too much on the earlier rooms, where everyone always gets clogged. Proceed, fairly directly, past the first garden scenes, past the many views of Charing Cross Bridge and the several views of the Houses of Parliament. They've got their points, I know, and it's nice that Monet was a lover of our London fogs, but there's much intensive viewing ahead. Keep moving, until you hit ponds.

I mean, specifically, not the pretty, delicate, circular and rather 18th- century-looking water-lilies pictures, but those in the next room, the more graphic and lurid ones, that somewhat suggest Edvard Munch. It's true we've just skipped almost half the show, but since almost the whole point of it is the last works, and since these lily pictures are first drafts for those infinite visions, they're a good start. They're in fact very clever (maybe not a word you'd associate with Monet). They perform an ingenious formal metamorphosis. A lot of it's in the cropping.

There's no bank in view. The pictures are all water, what's floating on it and what's reflected in it. And what's reflected is as boldly marked as what floats. Consequently they divide clearly into two flat, superimposed layers. It was 1907; no Western painting had done this before. The top layer is the water-lilies grouped in isolated flotillas on the pond's surface. The layer behind is the upside-down reflection of two shadowy treetops, with a brighter sky showing between and above (ie below) them. OK.

Now consider those lilies: where have you seen that formation of shapes before in paintings? In cloud studies, most obviously, with groups of strato-cumulus floating in a sky. And the upside-down trees-against-sky reflection: what does that configuration of light and dark look like? Chiefly, pictures of widening river-mouths, or rising springs, or waterfalls dropping into a pool. So between the two layers, image and shape change places. The water surface becomes open sky. The reflected sky becomes flowing water (as it were, a stream within a pond). Very neat, if quite subliminal, punning; but it's the basis of the last works' deeper spatial uncertainties.

Then it's the Venice pictures, which Monet himself thought a bit chocolate- boxy. Then his wife died, and he painted little for a couple of years. Then it's 1914, and suddenly he's painting on a much larger scale, and the brushwork gets magnified and emboldened with it. Here you find the first Monet painting where it's natural to ask: what's that meant to be? Monet probably wouldn't have considered Water-lilies, Reflections of Weeping Willows (1916-19) a finished work. His taste was neater than ours. But the point is, the space is starting to waver; the two layers - surface and reflection - merge in an overall milky-purple haze, mainly differentiated by rhythm and direction of brushstroke.

The most visionary images follow, ground-level views of the Rose Garden, and the Japanese Bridge with its overgrown canopy, where foliage burns in knotty, fireball coagulations of really thick paint, and the colours are frankly off the wall. A couple of these pictures may indeed register a disturbance in Monet's colour vision, due to cataract trouble. But he knew the colours of the world and he paints well enough for this still to be a doubtful explanation.

And now I'm hurrying to the Grandes Decorations in the last room, the giant lily-pond panoramas, two metres high, as much as six metres wide. Monet here develops this single motif into a space that's not abstract, but that resumes the whole of nature, with all nature's modes and elements in its repertoire. It can do boiling sunsets, lush meadow, dense mists, swelling ocean, pouring rain, the seabed. These amazing vistas disorient and dissolve between surface, depth, distance, atmosphere, flux, suspension, void. They put the viewer on the spot.

For if you imagine what viewing conditions the pictures demand, you see how odd they are. Size-wise, they look like public art, made to adorn a space with big walls, the sort of space you couldn't hope to have to yourself. But on the other hand, they're not for public viewing at all, not for sharing, or having an interesting discussion about in front of. The point of their size is the total immersion of the single viewer. There's this enormous panorama - for your eyes only. Think of mad King Ludwig having a full-dress performance of Lohengrin staged for him alone, an audience of one: that's about the size of the anomaly.

It might be simpler to say cinema. In the cinema, all audiences are audiences of ones. And when you enter the room, and see facing you the marvellous Water-lily Pond from New York, you may not think it, but your body knows at once what you're looking at: a wide screen. And as with a big movie, the impact is both overpowering and empowering. The vision is all yours to lose yourself in. But unlike a cinema, these paintings have only one true viewpoint - centre - in front of where the real depths sink in, with the encompassing vista balanced either side of you, and the pictures have to be hung at eye-level. You probably want to walk to and fro, approaching the canvas, backing off. But you don't want company, you want communion. There's no business so self-centred as self-loss.

Monet, of course, had bigger ideas about encompassing the viewer, and they're partly realised at the Orangerie in Paris. Personally, I've never thought this attempt at surround-vision really worked. Therefore I'm against the RA's imitating it, by putting its five panoramas all round the walls of a single, relatively small room. They need, ideally, one-to-one viewing. They need realistically as much space as possible, say a room each. (And I'm sorry to say the RA has actually taken three rooms out of circulation, for sponsors' parties.)

The experience, if you can get it, is the meaning. They're not about anything. I've made it sound rather spiritual, and that's all right if you see that with this art, the analogy can go either way - with the spiritual perhaps only a metaphor for the sensory. Or no distinction need be made. Monet once said he wanted a buoy for his coffin, to bob on the sea-surface eternally. But perhaps a better image for how the viewer is involved is the Buddhist idea of breaking the bottle, blending the water it contains with the ocean it's floating in.

Though I suppose, by the time you've squeezed your way through to the last room of "Monet in the 20th Century", you'll already know that feeling pretty well.

`Monet in the 20th Century', Royal Academy, Piccadilly, London W1; every day, to 18 April; admission pounds 9, concessions pounds 6, 0171-300 8000