Uffington White Horse (c.1000BC)

Things change. The ancient historian Plutarch mentions the ship of Theseus. After the hero's death, it was put on permanent display in Athens. Over time, its timbers rotted, and were replaced. Eventually they were all replaced, and "being so refitted and newly fashioned with strong plank, the ship afforded an example to the philosophers in their disputations concerning the identity of things that change". Some said it was still the same ship; others said it wasn't.

It sounds like a semantic question - it depends what you mean by "same". But it needn't exclude feeling. Later Athenian tourists might have been very disappointed to find that they couldn't touch a real bit of history. And if renovation involves alteration, too, it can become a highly sensitive issue. Think how bothered art-lovers get when restoration work goes too far.

In his lifetime, Giotto's most famous picture was a mosaic in St Peter's, known as the "Navicella" - the ship. Made around 1300, it showed the disciples of Jesus in a boat, in a storm, with Jesus standing on the water, lifting up St Peter as he sinks. If you go to Rome today, you can still visit this mosaic. But hardly anyone does, because it's been restored beyond all recognition.

In principle, a mosaic can be repaired pretty faithfully. It's made of pieces of coloured glass, and when one drops out it can be substituted. But that's not how the "Navicella" fared. It suffered centuries of disintegration and damage (due to being removed and relocated). When, in the late 17th century, it was put up where it now is, in the rebuilt St Peter's, it had been repaired so radically that almost nothing of the original design remained. You can see this by comparing it with a drawing made of its earlier state. You can see it in the style. Giotto's "Navicella" survives in name only.

Does that matter? It depends on how you value images. If you live in an art world like ours, where authorship and handiwork are prized, then the fate of the "Navicella" may seem tragic. As far as we're concerned, the image - Giotto's image - has ceased to exist. But if you live in an art world with different values, where images are prized above artists, you may disagree. Your priority will be to maintain an image in a functioning state, even if not in its authentic state.

Modern restoration policy favours authenticity. If an area of a fresco is completely lost, we don't replace it with our own guesswork. We leave it blank. Traditional restoration policy believed in maintenance. An image should be kept up. A picture cannot perform as a picture with a great blank in the middle of it. So if a part is lost, we should fill it in as best we can. On those terms, the renovators did the "Navicella" proud. They kept it going. There are some images, though, that don't just need a patch, now and again. They have to be kept up all the time.

The Uffington White Horse is the oldest of the chalk figures cut into the hills of southern England. Recent archaeology dates it to the Bronze Age, but its beginnings remain mysterious. It might have been made as a tribal banner, a battle memorial, a cult symbol, a horse traders' advertisement. The 100 metre-long image is only comprehensively viewable from the air, so how it was first marked out on the ground is a poser. With its beaked birdlike head, and its detached freeform limbs, it may look strangely familiar to the modern viewer - suggesting a Picasso graphic or a Matisse cut-out. But what form it originally took is another unknown.

There is no written record of it before the 12th century. There's no depiction of it before the 16th, and no informative likeness before the 18th. But the horse was never forgotten. The grass grows over it quickly. To be maintained, it must be scoured about every 10 years. Throughout its long history, the horse was kept going by regular renovation work, done by the local population.

There was no question, therefore, of preserving the original handiwork. In order to survive at all, the horse must be continually recut. And without any master blueprint, there wouldn't have been much chance of holding on to its original shape either. Over centuries of successive scourings, some kind of Chinese-whisper image mutation is unavoidable. Change is the condition of endurance.

The various drawings of the horse made in the 18th, 19th and 20th centuries suggest that its outlines drifted significantly, even during that short portion of its existence. The head in particular seems to have undergone drastic transformations. True, the horse is hard to view properly, and to a non-20th-century artist it would have seemed an alien shape, hard to draw. So these variations may partly reflect the vagaries of draughtsmanship.

Besides, unlike other chalk figures, this horse is not just a surface turf-cutting. Its makers laid down a metre-deep chalk-filled trench under it. Not all the scourings would have followed this "skeleton"; the trench's existence may not always have been known. But it exerted some control on the tendency to drift.

While the horse may have strayed down the centuries, it has certainly retained its identity better than the Westbury White Horse, in Wiltshire, which was wholly redesigned in a modern style in the late 18th century. The Uffington White Horse has probably never wandered very far from its first appearance. Still, what that was exactly, we can't tell.

It doesn't much matter. The Uffington White Horse is a classic case of a "maintained" image, where upkeep takes precedence over authenticity. There is the relay of scourings, linking back 3,000 years, but its original raison d'être is long lost. The figure is now preserved, as it was for most of its history, presumably, as a defining landmark, a notable antiquity. It's something that must be continually renewed - and in the process constantly modified - simply because it has always been there. It is the very image of a tradition.