The god of big things

It is now probably impossible to bring together what might be called the definitive Titian show. Talk of greatness, and of masterpieces, is one thing; exhibitions are another. More than 40 of Titian's works, from all periods of his life, are included in the new exhibition at London's National Gallery, and, tremendous though the show is, I found myself wishing for works that are not here, and also feeling that I could not escape the mores of my own time, the feeling that less sometimes really is more. But which less, which more?

Titian - Tiziano Vecellio - died 427 years ago in Venice, where he had lived and worked for most of his life. He was in his late 80s. He had been apprenticed to be a painter at around the age of 10 - under Giovanni Bellini - and continued working right to the end. He died the richest painter in Venice, and his career, posthumous as well as in life, has seen no falterings in his reputation. He is, it seems, unassailable, incomparable, and is regarded - in those circles where such things matter - as one of the greatest painters, not only of the Italian Renaissance, but of all time.

Titian's art was, in part, about making the unbelievable credible. This was his job. The weird scale jumps, the peculiar anatomies, the more you look at Titian, the more odd his human anatomies appear, the emphasis on colour, material and pictorial dynamism, the shifts in tone of voice within as well as between works, make looking at the artist truly exciting. We don't look to his paintings for commonplace truths, though truths they contain none the less. The apparent weight of Christ's body in The Entombment, the sense of presence in Titian's portraits, the differences between the coarseness of cloth and the luminous vulnerability of flesh, the length of the shadow cast by a man in a boat across the painted water. You want to grab hold of it all, because it feels as life feels.

As much as anything else, there's so much to look at in these paintings: the weather, the blue distances, the coastlines and heavy-leaved trees, the flocks of sheep and hunting parties and castles and spires and bluffs, the delicate, botanically accurate plants in the foregrounds, a twist of the brush that paints a salamander on a rock in some unregarded corner of a composition. The landscape is always ravishing. You get lost in there as you look beyond and between the humans and gods. The landscape is always a kind of dangerous invitation. And so is the particular paintedness of things, Titian's total scale of painted utterances.

The details are there to ensnare us. In The Bravo, one man grabs the collar of another. The second man, caught, twists his head, to see who has collared him. It is an exact moment of apprehension (and as such, perhaps a metaphor for painting itself). We follow the sleeve of the attacker, and get lost for a second in its expensive complications. This delays us. And then we find the hand emerging from that sleeve, which holds the hilt of the dagger, down in the painting's corner, as though the painter too is hiding the knife, allowing us to find it just before the victim does. This is a great example of Titian's sense of drama, how he slows action pictorially, how, like a conjuror or an illusionist, he misdirects us. But in Titian's art this is more than a trick. He is a great painter of activity and action. He manipulates the viewer's gaze, making us believe, even when things are curiously out of whack. In the Three Ages of Man there is a tiny flower in the foreground. It seems to pin the eye to the painting's surface. Deeper in the painting's space an old man sits on the grass, a human skull in each hand. The skulls are huge. I am drawn to the fact that the eye sockets are the same size as that flower. I miss the painting's main action. For now, this is enough, but it is also what makes us come back and look again.

There's no lack of drawing in Titian. He, it seems to me, understood that forms could be realised in different ways. The indeterminacy about where a form ends and where the space around it begins, a kind of disputed border between a thing - notably almost always a living person - and the space that contains it, gives his figures not just a sense of movement but of mutability and palpability. These figures breathe, pictorially. They are agents within the painting. They break out of - or shrink back from, or turn within - their boundaries. They seem, in fact, to be struggling against their allotted positions. The blurrings, the breaks in anatomy, the frankness of the painting (which declares as much as it often seems to obscure in Titian's later work) are to modern eyes like consciousness itself: intermittent, faltering, roaming about to get an unsteady fix on things. Titian may be telling us something we have always known, but was one of the first painters to be able to fully describe.

Generally, I am more drawn to the perpetual stillnesses of painting, than to dynamic and fluid compositions. Titian is a rare exception. The earlier of his two self-portraits here shows the artist, in his mid-50s, turned slightly away from the viewer. The hands appear unfinished. One hand is planted on his thigh, the other on the table where he sits. Two fingers of this hand are slightly raised from the table, as though he were drumming them, or caught in the thought that he'd like to be gone from here, as though in painting himself he has been held up, and would prefer to be elsewhere. His later self-portrait, aged about 70, has him in profile, with that hawk nose, again looking away. Vasari had it that Titian, after a long period of laborious work, would turn his paintings to the wall for months at a time, and when he looked at them again, he would regard them as though they were his mortal enemies.

In the final room of this exhibition, hung between the National Gallery's beautiful, late, and possibly unfinished Death of Actaeon (Diana pulls back the bow, the hounds set upon the hunted Actaeon) and the Fitzwilliam Museum, Cambridge's Tarquin and Lucretia (Tarquin's knee on the bed between her legs, knife raised to stab her) hangs what, over the past 20 years, has come to be regarded as possibly the greatest Titian of all, and one that was little known until it was shown in the Royal Academy's 1983 Genius of Venice exhibition. The "greatest works" game is futile, and the Flaying of Marsyas is, it must be said now, a painting among paintings, though it is deeply disturbing.

Its last showing here created a shock, not least because of its horrible subject - hung upside down, Marsyas is being flayed alive. Musicians play, Apollo with a knife is scraping away near one of his nipples, and another figure is skinning Marsyas's thighs. The painting is hung high, and we are on about the same level as Marsyas's head, looking him in the face. Between us and him is a little dog, lapping up the spilt blood. The dirty business isn't done, will never be done.

The canvas itself - like many later Titians - has a physical touch that also appears, in a sense, flayed. The artist's touch is, in parts, as violent as the subject. Painterly frankness becomes here almost brutally raw, though it is not consistent. Midas's gold crown, the knives themselves, the eye of the second, larger dog that pants in the corner, are painted with an absolute perspicacity. Marsyas stretched and dangling, allowing himself to sway and to be abused, is perhaps more detached than the viewer.

The painting has been seen (especially because Titian has painted himself into the picture) as a meditation on mortality and human suffering. But greatest painting? The show makes you want to travel, to see those other Titians that aren't here.

Scholarship nowadays, fights a rearguard action to uncover and preserve the artist's intentions, within the context of his times. Most of us are free to examine or to ignore such things: our reasons for looking may be woollier, or more personal, but merely hoping that some vague and notional idea of greatness is enough to make works meaningful for us is not enough.

Titian's progress is well documented here - his indebtedness to Bellini and to Giorgione, the development of Titian's signature style. Erwin Panofsky, tracking the trajectory of an artist's career, described the first, subservient phase during which the artist develops imitatively, in the manner of his teachers. Then a second period in which the artist struggles to find his own style, and, having developed it, moves into a final phase that, if not exactly cancelling what went before, certainly goes beyond it.

This, in our own time, is how late Titian has frequently been regarded. Charles Hope, writing in the exhibition catalogue here, is one of many wary historians for whom Titian's late work does not provide the kind of breaking of style we might like to imagine. A somewhat different Titian show could well make the continuity in Titian more immediately apparent. But what this exhibition does prove is Titian's range and adaptability, his endless curiosity about what is possible in painting, and how far he could go, and the ways in which subject and form constantly inform one another in painting, through the act of working itself. That is the continuous thread throughout his art, and his art's lesson.

· At the National Gallery, London WC2 (020-7747 2885), until May 18.