We'd build monuments to the discoverer of fire, if only we knew who it was.

The inventor of the wheel might get a plaque or two, if we had a name to chisel into them.

In the smaller world of fine art, the discovery of the bravura brush stroke is also an earth-shaking innovation. Luckily, we know who deserves credit for it: Tiziano Vecellio, the Venetian painter known simply as Titian. He's never really suffered from a lack of recognition.

During his long life, Titian's flamboyantly brushed pictures earned him massive fame, a sizable fortune and a knighthood from his friend the Holy Roman Emperor Charles V. After his death from plague in 1576, he got a lavish burial, the flattery of centuries of imitation and eventually an ornate monument over his tomb. And now he's getting a glorious retrospective at England's National Gallery.

The London show is drawing daunting crowds. But the best of Titian's works have such an overwhelming greatness that they can keep your full attention even when there's someone's elbow in your ribs. This survey provides a rare chance to come to grips with the extraordinary scope of Titian's creativity, from the lovely, serene, highly polished works he made as a young man to the frenetic energy of pictures made in his old age. With museums getting ever more cautious about shipping out their greatest works, there may not ever be another Titian spread like this.

There is only a tiny handful of European painters who rank with Titian in the range and subtlety of what they turned out: Rembrandt, Velazquez, Manet, maybe Cezanne. And not one of them could have achieved the stature that they did without standing on the shoulders of the Venetian giant. It is impossible to imagine the later history of Western art without the dabs of white, slashes of black and lashings of thick color that bear witness to the virtuoso work that builds so many of our pictures. And that means that whole history depends on having Titian at its start.

Of course, Titian had people he was indebted to as well. We don't know much about his early life -- we know that he was born to well-off parents in the Venetian village of Cadore, but not the year it happened. We do know, however, that by the early 1500s at the latest, he'd moved to Venice and taken up with leading painters there. The first rooms of this exhibition reveal his clear debt to his master Giovanni Bellini, who first established oil paint and canvas as the signature materials of Venetian art. Bellini used his oil paint in slick, seamless glazes to mimic how light travels through air, illuminates the objects that it finds there and then bounces from one surface to another.

That's where Titian began, too. The so-called "Gypsy Madonna" from Vienna, now attributed to Titian and dated to around 1511, shows an amazingly subtle observation of how light strikes and defines forms and their surfaces. Titian turns a pared-down image of the Virgin Mary and her child into flamboyant display, thanks to an elaborate silken cloth that hangs behind them on the right. The golden light that Titian's oils build around his figures seems to pay a kind of homage to the sacred status of this mother and child, but also to the material glory of the fabric that he's used to set them off.

The young Titian also took a page from his close colleague Giorgione, another of Bellini's genius followers. Only a half-dozen or so pictures by Giorgione survive; he died of plague in 1510 with his career just underway. Even in those few works, however, we can see him turning landscape into a special focus for high art, and using it to build a strange, moody stillness all around his figures. Sometimes, it seems that these Venetian pictures don't have any subject other than the mood they create around the unnamed figures shown in them. Rather than depicting specific characters from the sacred scenes or ancient myths that most Renaissance pictures illustrate, Giorgione's paintings -- and the Titians that followed from them -- sometimes seem to show just interesting human beings getting on with life.

Titian's "Gypsy Madonna" copies Giorgione in both the glowing sunset landscape that backs up the left half of the picture, and in the quiet, dusky mood that it sets up around Mary and her very human child. There are no halos or other divine attributes on show in this picture; you could read it simply as a lovely mother and her peaceful babe, with maybe a hint of melancholy that foreshadows pain.

Even in an early Titian picture that's more tightly based on Bible stories, showing a shepherd on one knee before the holy family, the most striking details are very down to earth: A lovely country sunset, a convincingly chubby baby, the pulled stitching in the kneeling shepherd's pants and the funny little water cask that hangs down from his belt. The supernatural is relegated to the far background, where a tiny angel is shown announcing the coming of the Messiah to the shepherd's mates.

But there's something else magical that we can just begin to glimpse in these very early Titians. For about the first time in the history of Western art, paint starts to take on an independent life.

The crisp white of the shepherd's untucked shirt, of the stitching in his spreading seam or of the wrappings around baby Jesus gets its special crispness from a web of thickly painted highlights that sparkle across these surfaces. Light catches on these flicks of paint just as it would on the kinks and folds in the textiles they depict. The same effects could perhaps have been achieved with a more old-fashioned, smoother surface. (Glossy photographs, after all, can render highlights fine.) But Titian found that thickly applied paint could do this work with special force.

In Titian, there's no longer a kind of point-by-point comparison of features in a picture with features in the world. So long as dabs of paint work as an effective surrogate for what we see in nature, that's all that matters; they don't need to mimic every tiny detail of the objects that they show. Titian discovers that you can translate the world into thick paint, rather than trying to render nature's idiom direct. Paint becomes a kind of strikingly effective metaphor for reality, rather than a close copy of it.

In Titian's gorgeous "Flora" -- the picture may be a pinup of a real Venetian beauty -- the half-dressed woman's golden hair looks so fine-spun, you imagine that it must have been painted with the smallest triple-zero brush. Then, looking closer, you realize that already by the beginning of the 1520s, Titian has learned to render even such fine-detail effects with broad swipes of his brush.

A decade or so later, Titian was asked to update a major painting that old Bellini had completed back in 1514 for the Duke of Ferrara's private chamber, the Camerino. The older picture now jarred with the more modern-looking works the Duke had commissioned from Titian for that same room. In this exhibition, the surviving Camerino paintings have been gathered in one place for the first time in 400 years. Despite the most atrocious, stagy spotlighting I have ever seen in a museum show, the display gives a clear sense of how far Titian had moved on in the few years since Bellini's contribution.

X-rays suggest that Bellini's "Feast of the Gods," on loan to London from our own National Gallery, must have started life with a striking sun setting on its horizon, as well as sunlight pouring in from the front left. When Titian set out to fix this inconsistent double lighting, as I believe was his intention, he made sure that the mountain landscape he stuck in to hide Bellini's sunset also showed off his novel painterly inventions. Dabs and flicks of gleaming paint pan out across its rocks and waterfalls; they signify light streaming in from the front left, but they also signal Titian's special new brushwork.

Of course, almost any painting breaks down into brush strokes if you come up close enough. Earlier artists, however, had mostly been at pains to hide this fact by building up as slick a surface as they could, even if it made their pictures look unnaturally stiff. Leonardo da Vinci, and then Giorgione taking off from him, had tried to undo this artificial-looking slickness through a kind of uniform soft-focus blurring of their paint. And then Titian came along, and realized that was wasted effort. It was fine to see a painter's brush strokes, so long as you could also read them as the thing that they were meant to represent. In amazing early Titian portraits such as his "Man with a Quilted Sleeve," an area of painted canvas can be all at once a mess of very visible brushwork and a gorgeous piece of silk.

In fact, it's more impressive to mimic reality in noticeable paint than to studiously avoid that striking contrast between the thing depicted and the stuff used to depict it. The more obvious the gulf between the world and the medium that effectively stands in for it, the more striking is that act of standing in. It can be more astonishing, that is, to look at an unlikely gob of paint and realize that it works to depict the world, than to look at trompe l'oeil slickness and barely even realize that it's painted.

The whole of Titian's career, you could argue, illustrates a constantly shifting balance between the work paint does to represent the world and what it does to show off its own good looks.

By the 1540s, Titian had reached the zenith of success, beyond what any painter had achieved before him: He was the favored painter of the doges and the Venetian state and aristocracy, of the popes and the most powerful mainland Italian nobles, of the Emperor Charles V and of his rich and powerful son Philip; he had a sinecure and pension bringing in a steady flow of funds, investments that helped to top up that income, and an eager market ready to grab at even mediocre portraits from his studio, or copies of the famous works he made for royal patrons. And at that point in his career, the balance between paint itself and what it shows is nearly even -- brush strokes are strikingly visible, but they also always seem to serve the subjects that they represent.

That even weighting is on clear display in the famous "Danae," from the Capodimonte Museum in Naples, which Titian painted between 1544 and 1546 for Cardinal Alessandro Farnese.

It shows the princess from classical myth just as she is being "visited" in bed by Jupiter, in disguise as a rain of golden coins. Alone and naked in her bedroom, glassy-eyed and staring into space, the lovely Danae sprawls with legs apart; the sheets are twisted in the fingers of her right hand; her left hand disappears between her thighs. (A Roman visitor, having seen this new work in the artist's studio, wrote back to the cardinal that Titian's earlier Venus of Urbino, whose hand is visibly buried in her crotch -- Mark Twain called it "the foulest, the vilest, the obscenest picture the world possesses" -- looked like a nun compared with this later bit of cheesecake.)

The golden shower of coins tumbling in mid-air comes as close as painting can to depicting orgasm itself, insisting on a striking equivalence between visual and physical sensations. And this crossing-over of one sense into another, known in the trade as synesthesia, continues across the entire surface of the picture, which is caressed by Titian's lightly loaded brush so that it conveys both the soft glints of skin and silks and spinning coins, and the tender emotions we're supposed to harbor toward them.

Toward the end of Titian's career, the balance between painted surface and the things it shows starts to shift more drastically in favor of the paint.

In 1571, Titian painted a "Tarquin and Lucretia" that is one of the most powerful images of rape I've ever seen. The virtuous Roman wife Lucretia weeps and struggles. King Tarquin, one stocking falling down in his animal excitement, uses his knee to pry her legs apart while brandishing a glinting knife. And Titian's frantic paint, swirling over Tarquin's gold-embroidered vest and sullying the bed's disordered sheets, seems to conjure up a notion of regal extravagance gone wrong.

Finally, in the years that led up to his death, Titian worked on his "Flaying of Marsyas." It's so coarsely painted that some scholars can't imagine that it's finished, even though it is signed.

The picture shows the torture death of the pipe-playing satyr Marsyas, who challenged Apollo to a music competition and then had to bear the god's choice of punishment when he lost. It now lives in the Archbishop's Palace in Kromeriz in the Czech Republic, but was brought out for this rare showing. The lighting in the London exhibition is so incompetent that it hides this painting's glories behind a haze of glare. But those who saw it well-lit 13 years ago in Washington, when the National Gallery mounted its own Titian extravaganza, must still remember them: Roiling paint everywhere across its many fleshy surfaces; a few striking moments of crisp detail, on the crown of King Midas and the viol of Orpheus (shown as witnesses to the event) and on Apollo's laurel wreath; and overall a very strange equivalence between glorious artistic intensity -- in Titian's brushwork, and in the musicmaking that it stands in for -- and pain.

The Titian exhibition is at London's National Gallery of Art through May 18. Call 011-44-020-7747-5898 or visit www.nationalgallery.org.uk

An early Titian work, "The Gypsy Madonna," reveals his subtle observation of how light strikes and defines forms and their surfaces . . .. . . while "Tarquin and Lucretia," painted late in his career, reflects a bolder use of paint and conjures the notion of regal extravagance gone terribly wrong.Updating Renaissance art: Giovanni Bellini's "Feast of the Gods," reworked by Titian, signals the latter's new brushwork and its stunning rendering of light."Danae," from 1544-46, reflects Titian's late-career shift toward achieving an equal balance between the paint itself and what it shows."Flora," from 1515-20: Rendering fine-detail effects with broad swipes of the brush."Man With a Quilted Sleeve": Making brush strokes visible -- and worthy of appreciation.