Now that graffiti artist Banksy has been “outed” as a former public schoolboy, the market for his stencil-based pieces will inevitably decline. Talk to hardcore “graff” practitioners and fans, and you quickly realise that his credibility dwindled long ago, around the same time that prices for his work started to rocket upwards in auction houses around the world.

Context is key: Banksy’s art tunnel in Waterloo

But if reports are true that this supposedly anarchic prophet of the streets is actually a nice middle-class bloke from Bristol, his popularity among a more mainstream audience is surely about to crumble, too.

It will, however, crumble for the wrong reasons. I don’t have a problem with the idea that a middle-class kid can become a graffiti artist. Who cares about Banksy’s background as long as what he creates is good? Banksy is incredibly popular, and rightly so. When seen in situ in city streets, his work feels aesthetically snappy and brilliantly witty. At his best, he creates joyful, eye-catching interventions in otherwise drab urban spaces that can cheer up almost anyone.

But, like all street art, Banksy is entirely about context. Put his work inside an art gallery, and suddenly it seems insipid, two-dimensional, even moronic. Street art is about grabbing the attention of passers-by, about getting across a message in an otherwise image-saturated concrete landscape. But it rarely repays close scrutiny, just as a one-line joke has less to offer than a complex novel. In short, Banksy makes art for people who don’t like going to art galleries.

If you don’t believe me, just look at his canvas The Rude Lord, an 18th-century painting altered so that its country-squire subject sticks up his middle finger at the viewer. It sold at Sotheby’s in London in 2007 for a record £322,900. I would love to discover who was prepared to spend so much money on something artistically so slight (especially when so much of Banksy’s work can be seen up and down the country for free). What could you possibly get out of such a painting after looking at it above the fireplace for the umpteenth time?

More than a year ago, I wrote a piece for the Telegraph asking whether street art’s enfant terrible had sold out. My conclusion? Well, put it this way: when I heard that people cheered after one of Banksy’s canvases failed to sell at an evening auction at Sotheby’s in New York earlier this year, inwardly I rejoiced. People often talk about the madness of the art world, but to me, Banksy’s rip-roaring success at auction seemed like the maddest thing of all.

Hadrian: a titan of antiquity

A magnificent show at the British Museum brings to life an enigmatic figure of the ancient world as never before, says Alastair Sooke

It’s easy to assume that ancient history is, well, ancient history, the dusty province of antiquarians poring over old texts and artefacts. What more is there to learn about classical Greece and Rome?

	Presiding spirit: a colossal marble head of Hadrian, which was dug up in Turkey last year

Think again. The British Museum’s magisterial new exhibition about the life and times of Publius Aelius Hadrianus, ruler of one of the mightiest empires the world has ever seen, reveals the extent to which our understanding of the ancient world is fashioned from fragments of historical evidence.

It also reveals how it is constantly evolving, as hitherto unknown objects are discovered. Ancient history may be the stuff of marble statuary, but, it turns out, it is anything but set in stone.

Take one of the show’s highlights, a colossal marble head of Hadrian spotlighted as you enter the Reading Room, where the exhibition is held. It was dug up last August in south-west Turkey, like a whopping great gemstone plucked from the earth’s crust.

Once the apex of a statue 5m tall, Hadrian’s head now looms above visitors like the presiding spirit of the entire show. It is a gleaming wonder, intricately chiselled, and as valuable-looking as an oversized Fabergé egg. Looking at it is as close to coming face-to-face with a Roman emperor as it’s possible to get.

Hadrian came to power in AD 117. He inherited an empire that extended across three continents, from the wild Caledonian lowlands in the north to the Sahara Desert in the south and to the River Euphrates in the Middle East. His first act was to withdraw Roman soldiers from Mesopotamia, or modern-day Iraq, so earning a reputation as a lover of peace. It is startling and saddening to discover that the conflict zones of his empire are the same as ours today.

Over the following two decades, until his death in AD 138, he consolidated the Pax Romana.

He built walls at the empire’s frontiers, including the famous rampart storming 80 miles across northern England. He embarked on a prodigious building programme, and many of his monuments, such as the Pantheon and his Mausoleum in Rome, still stand today.

He showered riches on Greek cities to ensure that the eastern reaches of his empire would form a bulwark against the barbarians beyond. In doing so, he laid the foundations for the Byzantine Empire, which lasted until 1453.

He is also famous for being an aesthete, passionate hunter and philhellene, as well as for his infatuation with Antinous, his young Greek lover, who mysteriously drowned in the Nile in AD 130, and whom Hadrian deified after his death. Hadrian’s sex life was the subject of scurrilous rumours long after his reign and remains a piquant area of speculation today.

But, if anything, this exhibition suggests that getting a handle on his character is impossible. If you don’t believe me, just look at the different types of full-length portraiture by which his image was propagated through the empire (you can always tell a portrait of Hadrian by a strange physiognomic quirk: deep diagonal creases in both ear lobes).

The most celebrated example is the statue discovered in the ancient city of Cyrene in northern Africa in 1861. Painstakingly pieced back together by the Victorians, it presents Hadrian wearing a distinctive Greek mantle rather than a Roman toga, and has been illustrated in books about the emperor ever since.

Recently, however, the thick 19th-century plaster was chipped away, and it was discovered that Hadrian’s head never belonged to this paunchy body. The Victorians so wanted to believe in their image of Hadrian as a peaceful champion of Greek culture that they wrongly reconstructed the shattered fragments to fit their preconceptions.

Much more convincing is the frightening image of the emperor in warrior mode, squishing a vanquished barbarian underfoot. Hadrian was a military man to the hilt, a hardened career officer stationed in Syria when he learned of Trajan’s death. Forget the enlightened prince of peace: Hadrian’s role as a ruthless warlord is one we should get to know. His brutal repression of the Jewish revolt in Judaea in AD 132 is unlikely to have been the only instance of his imperial ire - it’s just the one that historians know most about.

Housed beneath the resplendent dome of the Reading Room, itself modelled on the Pantheon’s vast concrete cupola, this exhibition might not be as spectacular as the recent show in the same space devoted to China’s first emperor, but it is still magnificent. On display is a glistering hoard of more than 180 objects, including stunning statues, as well as exquisite works of art from Hadrian’s sprawling villa complex at Tivoli to the east of Rome. There are gleaming coins and cameos, ornate silver cups, pilaster capitals, and a shimmering glass bowl.

And, of course, there are familiar objects from the museum’s collection, such as the bronze head of the emperor dredged up from the River Thames in 1834, with its skew-whiff features and pointy Spock ears. No exhibition in Britain will ever again offer such a complete portrait of life under Hadrian. The lucid and authoritative catalogue, written by the show’s curator Thorsten Opper, is a joy.

More than anything, though, this show speaks of the eternal enigma of the past. Remember: the sources about Hadrian’s life were written long after his death and are devilishly unreliable. Imagine how little we would know if our only written records of, say, Elizabeth I dated from the 19th century. And, if it weren’t for the odd sliver of papyrus preserved in the African deserts, large swathes of our knowledge about Hadrian would evaporate.

Some of these fragile scraps are included in the exhibition. Cracked and delicate, and covered in scratchy ink, they look as though they are about to shrivel to dust before our eyes. How humbling and moving to be reminded that our connection to the past is quite so tenuous.