Thomas Sutcliffe: Return of a man called horse

That he brought it on himself wouldn't be any kind of consolation. He had to cope with that knowledge when he was still alive, and it pricked him like a burr caught between saddle and skin. "Mr S repents much his having established a character for himself," his friend Josiah Wedgewood wrote, "I mean that of horse painter, & wishes to be considered as an history & portrait painter."

The problem for Stubbs was that he spotted a niche in the market, wedged himself into it with enormous energy and talent, and then found it virtually impossible to wriggle out again. One of the things you learn from the National Gallery show is that Stubbs cashed in on the horse-racing craze by establishing a kind of horsey hall of fame, in Conduit Street, called The Turf Gallery, which featured paintings of famous racehorses. If they'd had T-shirts in those days, there's little doubt that he would have produced those, too.

A lot of admirers have been tugging hard to pull Stubbs free of the horses ever since - and they've done pretty well. And despite the fact that the National Gallery has harnessed up a whole stud-farm of horses to pull back in the other direction, even in this exhibition it's possible to see what makes Stubbs such a distinctive and durable painter.

Oddly enough, that is even true of the paintings in the exhibition in which absolutely nothing is depicted but horses - namely the frieze-like paintings Mares and Foals, in which the main subjects are seen against a plain greeny-beige background, marked only by vestigial flecks of shadow at the hooves. These would seem to be the ultimate distillation of the art of the horse - pictures which suggest that anything other than a nag can safely be dispensed with as mere distraction from the only interesting theme. But what they help alert you to is Stubbs's singular fastidiousness about outline. What makes him stand out as a painter is the curious charged quality of his paintings - described as "surreal" or "enigmatic" - and it comes from the way in which the subjects sit against the background.

Looking at a Stubbs painting, you often feel that you could get your thumbnail under the foreground objects and lift them free of the picture, like those moveable plastic stickers that children arrange on an empty background. His main subjects live under a different light from everything else in the picture - whether they're human or equine or canine - and their colours have the extra crispness of something designed to take precedence over its surroundings. This is emphasised by the way he displays an almost childish reluctance to let anything overlap the main subject. He will paint a clump of weeds with such botanical exactness that you can identify each plant, but he won't allow a blade of grass or a wisp of straw to get in the way of a horse's outline.

Even in a picture such as Gimcrack on Newmarket Heath, with a Trainer, a Stable-lad and a Jockey, in which the great racehorse is shown being rubbed down with straw after a triumph, and in which mere truth to nature might demand a bit of occlusion, the outer contour of the horse is left unviolated, often by a razor's breadth. The artist will overlap a horse with another horse - or with its human attendant - but almost never by the general bric-a-brac with which he fills the foreground of the picture.

There are hard-nosed, non-aesthetic reasons for this. If you have commissioned a celebration of a particular horse you don't want that expensive profile, every curve and angle of which testifies to the excellence of the bloodline, to be interrupted by dock-leaves. And it's notable that the exceptions to the rule occur in Stubbs's pictures of haymaking, or of horses being attacked by lions, where the identity of the individual horse is less crucial.

But whatever the motive for the unadulterated nature of Stubbs's delineation, the result is an utterly distinctive aura to his images. The pictures shimmer between the scientific and the magical - between a sort of ruthlessness and a helpless submission. The only thing that blurs their wonderful precision, in fact, is the fact that they're pictures of bloody horses...