The Critics: Exhibitions: Minor? He was huge in the 15th century

Rogier van der Weyden National Gallery, London

You can't help feeling sorry for Rogier van der Weyden. In his lifetime - 1399-1464 or thereabouts - the Netherlandish painter was among the first artists in Europe, lionised not just by his fellow Flemings but by Alfonso V of Naples and the Viscontis of Milan. His influence on Italian painting was incalculable. Florentine humanists called him "the glory among painters"; one cardinal, "the greatest of artists".

By the time Vasari published his Lives a century after van der Weyden's death, all that had changed. Now, the "glory among painters" was merely one of Vasari's "divers Flemyngs", only included at all as a footnote to the careers of two of his countrymen, Jan van Eyck and Hans Memling. According to Vasari, an elderly van Eyck, having invented the technique of painting in oils, "decided to impart it to Roger of Bruges, his pupil, who passed it on to his disciple, Hans". That, as far as Rogier van der Weyden and the Lives go, is that. Just to rub salt in the wound, van der Weyden had never been van Eyck's pupil. Sic transit gloria pictores.

What had happened in the intervening hundred years? In a word, naturalism - that is to say, a belief that the prime duty of a painter was to reproduce life in the most visually credible way. Since naturalism was what van Eyck's painting was all about - think of the mirror in the Arnolfini Marriage - it was all Jan this and Jan that. Van der Weyden, who had adopted a style that looked back to the highly stylised world of Gothic art, had come to be seen as unpardonably anti-natural and old fashioned by comparison. To a large extent, the view has stuck.

Visit the National Gallery's new exhibition of van der Weyden's work, however, and you will see the limitations of that view. Hardly a block- buster - the show includes the gallery's own five pictures with half a dozen more from the Getty Museum and the Gulbenkian Foundation in Lisbon - the small scale of the National's exhibition is just about right for appreciating the extraordinary refinement of van der Weyden's art.

A good place to start is with the London Piet, now attributed to the master's workshop rather than directly to his own hand. The first thing that will strike you about the picture - especially if you have passed by the almost contemporaneous Arnolfini Marriage on the way - is the obvious medievalism of its composition. The body of the dead Christ is pushed right up against the picture plane, turned towards the viewer rather than to the grieving Virgin; there is no sense of nature in his posture, nor of dead weight in the delicate embrace with which his mother supports him. Christ's loincloth and the drapery of the Virgin's dress make no attempt to suggest the bodies beneath them: they are all about surface pattern, calligraphic form. Most strikingly, there is a clear disjunction in the picture between the anti-perspectival Golgotha of the foreground and the highly naturalistic landscape that stretches out behind it.

This last quality is particularly worth noting, since it suggests that Rogier didn't choose not to paint naturalistically because he couldn't. Rather, there is a hierarchy of styles in his painting. Naturalism, mere naturalism, is for the temporal world; in the spiritual world of the Crucifixion, rules can (indeed, must) be broken, to show that natural laws do not apply there. Somewhere in between these two is the world of the praying donor, who van der Weyden uniquely places in the same pictorial space as the central drama of the piet, but who is none the less clearly not a part of it. In the theological terms with which every Renaissance art-buyer would have been familiar, the donor is in that halfway house of the prayerful known as a state of grace.

The Getty's Dream of Pope Sergius takes this tendency further. In the Life of St Hubert series of which it forms a part, an angel appears to the Pope to order him to make the saint Bishop of Maastricht. In van der Weyden's version of events, Sergius kneels at his prie-dieu in the bottom left foreground, hurries out of the house (ignoring a pair of petitioners in his haste) via an adjoining door, crosses a bridge over the Tiber in the painting's mid-ground and arrives at St Peter's in its top right background. Even allowing for miracles, having the same person appear four times at four different moments and in four different places in one picture might stretch credulity: so van der Weyden gets around this by containing the conflated time of his story in an equally miraculous conflation of space. The perspectival depth of the painting is impossibly vertiginous, leaving the viewer with the faintly Lewis Carrollish sense of being about to fall into it. Had van der Weyden plumped for a naturalistic style, Pope Sergius would have been a dog: as it is, it is a masterpiece. The picture's detailing may be Gothic, but there is nothing old- fashioned about its inventiveness.

Nor is there anything backward-looking in its portrayal of human emotion. In the words of Erwin Panofsky, the great Netherlandish scholar, van der Weyden's logic was that of reculer pour mieux sauter - step backwards in order to take a better jump forwards.

Nowhere is this truer than in the National's Magdalen Reading, shown here for the first time (since it was dismembered by anti-Catholic iconoclasts) with other parts of the altarpiece to which it originally belonged. Freed from the demands of naturalism, the mind of the converted Magdalen is instead shown by the contrast between the rucked-up richness of her skirts and the supreme poise of her gesture. The two fingers with which she turns the page before having finished it say more about her inner life than any huge gesture of a Caravaggio. It is a little piece of genius.

Rogier van der Weyden: National Gallery, WC2 (0171 839 3321), to 4 July.