ART / False impressions: Andrew Graham-Dixon on the sunshine and loneliness of Alfred Sisley, 'the club secretary of Impressionism', at the Royal Academy

PISSARRO called Sisley 'the typical Impressionist' but this has come to seem more of a weakness than a strength. While his colleagues in the movement experimented with other styles, other subjects, Sisley remained the solid, reliable club secretary of Impressionism. His life sometimes seems to have been one long riverbank excursion, devoted to the memorialising of one transitory 'impression' after another. Sticking to his plein-air strategies, Sisley continued to register strength of breeze, agitation of foliage and water, disposition of clouds, right to the end.

Thirty years after Sisley's death, H R Wilenski wrote an article called 'The Sisley Compromise' in which he rested the case for the prosecution: 'Sisley was never sensational or vulgar. He kept the tones and planes in excellent relation. He had sensibility, good taste and pictorial tact. But he was not a great and original master; he was not a Monet, a Seurat, a Degas or a Renoir.' It is hard to think of a better demonstration of how to damn someone with faint praise.

'Alfred Sisley', at the Royal Academy, sets out to reopen the Sisley case, to prove that he was a more complicated artist than his reputation might suggest. Sisley's preference for pastoral over urban subjects, it is argued in the catalogue, gives his art 'a particular relevance for us today'. He is cast as an artist whose concerns foreshadow those of the contemporary environmentalist lobby. You could as plausibly (or implausibly) argue the case for just about every landscape painter in history but Sisley is a fascinating artist, precisely because he is the most typically Impressionist of the Impressionists. His painting demonstrates, with unusual clarity, the peculiar problems and anxieties implicit in such a reputedly carefree approach to painting.

The earliest works in the RA's exhibition show Sisley starting out as a painter of lyrical, wistfully poetic landscapes in the manner of Corot, and as a painter of densely worked scenes of village life patently indebted to Courbet. The tonality of these works is almost overbearingly dark and their mood is equally sombre and austere. Village Street at Marlotte, near Fontainebleau is a dour little painting of a labourer chopping wood under a cloudy sky. The woodcutter, whose face has been left blank, is painted more as a thing than a person, and he seems hemmed in by the flinty world of stone walls and squat, heavy houses that he occupies.

There are no people at all in another early work, Avenue of Chestnut Trees near La Celle-Saint-Cloud, in which the artist contemplates the dark vista down an overgrown woodland track with only a nervous deer, in the middle distance, for company. This is a sullen, late Romantic painting, which pictures a natural world that seems hostile or at best indifferent to man. Sisley's melancholia may seem to have been dissipated in his later work, where the vigour of his Impressionist technique can easily pass for a form of happy-go-luckiness. But maybe it does not, ever, entirely go away. It just takes a different form.

The Bridge of Villeneuve-la-Garenne is one of Sisley's best-known Impressionist paintings of the early 1870s. It is a sunny, apparently untroubled picture of just the kind that has made Impressionist art so enduringly popular, so appealing to the escapist sensibility. Under a blue sky across which a few wispy clouds are scattered, Sisley notes the sun-struck creaminess of the white house on the riverbank opposite him, the pair of women on holiday taking a boat-trip down river and, in the shade of the stone and cast-iron bridge next to which he has set up his easel, a couple enjoying a picnic. A pleasant enough picture, painted with brio: just the sort of thing that might confirm Sisley's stature as an enjoyable but unchallenging artist of the later 19th century.

Yet this exhibition also suggests that Sisley's concerns ran deeper than the merely literal. As you progress through the show, you begin to notice how often Sisley places himself at a distance from the scenes which he observes, how frequently he interposes a barrier of one kind or another - sometimes a river, sometimes a patch of waste ground, sometimes a stretch of empty road - between himself and the subjects of his gaze. It could be argued that this habit is merely the manifestation of Sisley's compositional preferences, that distance enabled him to frame a view as he pleased. But there may be more to it than that.

Throughout Sisley's oeuvre, there is a strong sense of the artist as someone peculiarly aside or apart from his own subject matter. Physical distance reads as a metaphor for emotional distance. This is most apparent in his many paintings of light industrial labour set along the canals and rivers near Paris. In The Seine at Port-Marly: Heaps of Sand, Sisley's view of the men in their boats, dredging the river, is partially obscured by two mooring poles, while the right side of his painting is occupied by the object of their labour, the heaps of sand of the title. The painting sets up an implicit contrast between the useless activity - art - of which it is the result, and the busy scene of productive labour which it records. You wonder quite what the artist was doing here, and why he felt impelled to paint the subject. How, exactly, does the painter fit in?

That question is never quite answered. The involuntary subject of Sisley's art might be said to be the artist's own sense of his own rootlessness, and a consequent uncertainty of purpose. Sisley's freshness of response, reflected in the dabbed, flickering skeins of paint with which he improvised his visions of the world, can seem wearying, precisely because of this. Sisley gets the job done, over and over, but still you are left wondering what it all adds up to, what purpose these views of riverbank idylls, of parks and gardens and fields might have. Optical truth to the facts of nature, the usual Impressionist justification, will not quite do.

If Sisley is the quintessential Impressionist, the lesson of his art is that Impressionism naturally creates, in its practitioners, an uncertainty of purpose and a sense of never quite belonging in the world. The idea that art should consist of an infinitely sustainable sequence of 'impressions', relayed in paint, is implicitly alienating. It defines the artist as a kind of mechanical sensorium whose job it is to record the optical phenomena which he encounters; and it means that the artist will always be confronting a world which his own activity (which is to look and note, rather than to participate) separates him from.

It is no coincidence that Impressionism's favoured subjects are the picnic and the daytrip. These are the leisured, respectable forms of Impressionism's true subjects, which are transience and vagrancy. Impressionism condemns its artists to be constantly on the move, constantly seeking out new, ephemeral 'impressions' to paint. Many of Sisley's pictures covertly acknowledge this by focusing on roads or bridges, paths or rivers: these are emblems of the Impressionist's own activity, which is perpetual travel, a constant uprooting of the self in search of subject matter.

Despite the general sunniness of the scenes Sisley painted, his art is shot through with an unusual, subtle form of melancholy, a sense of sad abstractedness from the world re-created in paint. Just occasionally, this comes to the surface in works that have an almost Symbolist undercurrent of mystery and alienation: Snow at Louveciennes, which pictures the small, distant figure of a woman walking away from the painter down a snow-muffled village street, is a work whose subject seems to be, precisely, not knowing another person, not knowing where they are going or why. But much the same sense of incomprehension and distance is implicit in more cheerful works like Villeneuve-la- Garenne, where you find the painter, on one side of the river, all alone, contemplating a number of large and graceful summer residences lit by afternoon sunshine on the riverbank opposite. Figures wander up and down, but far away: the painter is a voyeur, peeping through the keyhole of lives he cannot share.

This may explain Sisley's anxious habit of thoroughness, his tendency - manifest, most conspicuously, when he visited Hampton Court in 1874 - to paint series of pictures that amount to visual catalogues of entire locales. Sisley, here, is like the tourist who compensates for the knowledge that he has not really known a place by taking photographs of everything in sight. Painting, for Sisley, becomes just such a retrospective proof of presence, a confirmation that, yes, he actually was there. But the paintings also betray, whether involuntarily or otherwise, the loneliness of the solitary tourist Impressionism makes of the painter, his sense of apartness from the experience he is having. Hampton Court Bridge: the Castle Inn is predicated on a void, a large area of sand-coloured path whose emptiness is relieved only by a few, self- possessed pedestrians studiously ignoring the artist.

Impressionism, in the work of its most typical practitioner, could even be said to be existentialism before the fact: after all life, according to Impressionist theory, is converted into a disconnected sequence of meaningless visual experiences. But maybe it is more accurate to say that Impressionism is existentialism in disguise, since if it presents a world that is merely an agglomeration of optical facts, unburdened by religious or moral or social significance, it also pretends not to worry about the implications of such a conception of things.

This may, in a roundabout way, explain the intensity of what are among Sisley's last paintings, his sequence of views of The Church of Notre-Dame at Moret-sur-Loing. They are directly comparable with Monet's famous series of paintings of Rouen Cathedral, although what they attempt is, in fact, the precise opposite. Monet famously remarked that 'the motif for me is insignificant - what I want to reproduce is what there is between the motif and myself', and he painted the facade of the cathedral accordingly, registering it not as substance but as a kind of infinitely malleable, almost molten plasma, subject to infinite variability according to the weather, the season and the time of day. This was, perhaps, the ultimate demonstration of the Impressionist sense of the insubstantiality and ephemerality of all things.

But although Sisley also painted the church at Moret on many different occasions and under very different conditions, what comes across most vividly from his pictures is not the volatility and mutability of the motif but its tremendous, craggy solidity, its presence as an unchanging physical fact. Sisley even reneges, here, on his characteristic style, developing a technique much heavier, more substantial and impasted. The associations of his subject may not be irrelevant. Perhaps the church represented, for Sisley, everything that his Impressionist precepts denied him: a sense of firm purpose in the world, a sense of community, a sense of continuity and, above all, of enduring, and belonging, in just one place. The church at Moret, painted by Sisley, becomes more than itself. It is a compensating fantasy: of permanence, of inhabiting rather than touring the world.

(Photographs omitted)