SCULPTURE / The texture of memory: Andrew Graham-Dixon on the ghostly 'memory' sculptures of Medardo Rosso on show at the Whitechapel gallery in London

A baffled critic, writing in 1904, described the art of Medardo Rosso as 'sculpture that no longer resembles anything, not even sculpture'. The remark was meant disparagingly but Rosso may have taken it as a compliment. Simple resemblance to things was not, to his way of thinking, much to be desired. A Rosso sculpture is a solid paradox: a monument to evanescence; a thing that exists to challenge the notion that anything or anyone can pretend to be more than fleetingly substantial. Rosso made works of art that do, in fact, resemble things seen in the real world, but with deliberate, melting tenuousness. The likeness is always threatened and unstable, as though it might just have been produced by a trick of the light.

Medardo Rosso is generally less well known than he should be and virtually unknown in this country, a state of affairs which the small but invigorating exhibition of his work currently occupying the upper floor of the Whitechapel Art Gallery aims to correct. Rosso, who was born in Turin in 1858 but spent most of his creative life in Paris, has been remembered somewhat cursorily by art history as the Impressionist of sculpture. This is partially true, but one of the chief virtues of this exhibition is the extent to which it complicates such a judgement and makes, of Rosso, much more than a footnote.

To look at one of Rosso's figurative sculptures is to see a momentarily palpable ghost, an apparition that looks as though it has suddenly materialised rather than been made. The last of his sculptures, Ecce Puer, is a child's face made to look as if accidentally formed from lava: the likeness of a human being preserved but vulnerably so, on the brink of melting back and away into the nothing from which it came. The image itself, like many of Rosso's images, is fairly conventional - his ghostly young boys and pallid women are the stereotypes of Symbolist painting transmuted into sculpture - but what counts is the quality of its making. Rosso's achievement was to bring to sculpture a new language, full of visionary possibility. His deliquescent, almost insubstantial forms showed other sculptors (Picasso among them) that the indeterminate and changeable qualities of the visible world, so long the exclusive property of painters, might also be expressed in three dimensions.

Rosso's early sculptures are faintly irritating in their evident ambition to rouse the viewer to sympathy and pity: the laughing waif that is Rosso's Ragamuffin, an Artful Dodger cast in bronze, the cackling Procuress and toothlessly grinning Old Man are too much the stock figures of the underclass, too cliched a composite image of the poor, to touch the heart as Rosso presumably intended them to. He was not cut out to be a social realist. But these sculptures already contain omens of the later work: the roughness of their edges, the wavering contour with which Rosso models flesh (which recalls Daumier as well as the art of the Rococo), will soon be put to new uses.

'The impression you produce on me,' Rosso wrote to a friend, 'is not the same if I happen to see you alone in a garden, or amongst others in a salon, or in a street.' Rosso's ambition was to make sculpture that could be true to the mobile, fluctuant nature of experience: true to the fact that no one and nothing ever looks the same twice; and true, too, to the way in which experience is always refracted and altered by the distortions of memory. He began to make sculptures that combine a form of extreme precision with a form of disconcerting vagueness, as if he was attempting to recreate the quality of a memory in three dimensions. The small wax sculpture that he called Sick Man in the Hospital is exemplary. Its precision lies in the inclination of the modelled body, the way in which Rosso has recalled the hunched slump of an invalid in the angles and juxtapositions of the forms; yet the surface of the sculpture is elusive and slippery, molten, uninformative about the minutiae of the man's appearance. It is not strictly speaking the sculpture of a person but the sculpture of a memory of a person. It contains, like a memory, the essence of something seen, the recollection of how a man sat, ill and disconsolate, in his chair - but leaves everything else unfocused.

Rosso's preference was always for soft forms, forms made out of material so yielding and fragile that they seem perpetually on the edge of dissolution. His most innovative sculptures were made out of wax, which in Rosso's hands assumed the properties of a comestible substance. Impression on the Boulevard: Lady with the Veil is the likeness of a woman in a hat just discernible in a great yellowy-green chunk of wax, veined like a piece of Gorgonzola. Jewish Boy is the head of a child, solemn and impassive, that looks as though it might have been modelled in butter. Many of the sculptures have this unstable and faintly unpleasant organic quality, works of art that look as though they need to be refrigerated or they will decay: Sick Child, made from a particularly distressed piece of bright orange wax, is the drooping inclined head of an infant that has the slightly sticky sheen and tactile properties of heavy custard.

Rosso invented a new and resolutely anti-monumental form of sculpture that he clearly believed was truer to the way we are than other and older forms of art. This exhibition includes the disparaging and somewhat clumsy copies that he made of Donatello's David and Michelangelo's Medici Madonna, which were designed to demonstrate the superior lifelikeness of his own more volatile works. The truth is that Rosso's techniques did not produce sculptures that were demonstrably more like reality than those of the old masters, but sculptures that were truer to his own notion of what it means to be alive. Most of his innovations as a sculptor served to heighten his essentially morbid view of life. Rosso's subjects were often the diseased or the dying, and the sculptures that he made of them are, themselves, sickly and frail objects, poised between being and non-being. Rosso's sculpted equivalents for people are crumbling or melting things, objects that derive their pathos from their fragility.

Rosso made almost no new sculptures during the last 20 years of his life, when he seems to have devoted most of his energy to having his existing work photographed in ways that pleased him. This suggests a lack of self-confidence, a fear that, unless people could be trained to see the work as he saw it, it might be perpetually misunderstood. Rosso had his sculptures photographed to look like spirits emerging from circumambient darkness and in the most striking photograph that survives of the man himself, he looks like one of his own creations: a white-faced ghost, his face a blur, a likeness only half caught because caught in motion. Photography would, in the end, kill him: he died from blood poisoning after dropping a box of photographic plates on his foot. 'We are nothing but a play of light,' he once remarked. 'Nothing is material.'

(Photographs omitted)