TITIAN. National Gallery, London, to May 18. www.nationalgallery.org.uk

Michelangelo was pretty impressed by Titian but - being a Florentine - no way could he praise a Venetian painter unreservedly. According to Vasari (whose Lives of the Artists put Florence on a par with Mount Parnassus), Titian had “a fine spirit and a lively and entrancing style”; a shame he hadn’t learned to draw well and paint “with more method”.

To which, looking at the 40 or so Titians at the National Gallery, it’s tempting to respond with a superior smile. Michelangelo didn’t know that Titian was to be regarded as perhaps the greatest painter of all time, and that the variety of his methods was to be supremely influential.

As for the “fine spirit” - Titian could also be caustic. The portrait of Pope Paul III (who, incidentally, commissioned Michelangelo’s “Last Judgment” for the Sistine Chapel), shows His Holiness seated in massive mulberry-coloured robes, a wily old tortoise, a shrewd confidence trickster finally caught out.

The spirit pervading the exhibition is one of ardent concern for the rhythms of nature and the patterns of existence, a spirit of transformation involving everything from trees and spaniels to dimpled knees and chafed papal velvets. The paintings thicken as the decades pass and miracles happen, not simply in subject matter - Jupiter transubstantiating himself into a shower of coins, splattering a naked Dana - but in the handling.

Titian’s paint is the stuff of metamorphoses; one wipe and the skin peels, a dash of white and the dagger glints.

Sensuality improves almost every occasion. Obviously so in Dana smilingly receiving Jupiter’s golden tribute, or a Flora practised in the art of letting the robe slip to reveal shoulder and breast. The sexiness doesn’t have to be spelt out. It’s in every touch, even in the hands-off of “Noli Me Tangere”, in which the risen Christ swerves to avoid Mary Magdalene’s outstretched hand.

It’s good to know something of what is involved in each painting beyond such obvious gestures. For example, you can tell by looking at him, hunched in his chair, that Paul III was the sort of Pope who considered everybody a threat; but you should also know that the preceding picture, a half-length portrait of young Ranuccio Farnese, the eternal male adolescent, is of one of the papal grandsons.

The more elaborate and intricate compositions are bound to be mysteries to anyone unversed in myth or gospel. “The Feast of the Gods” is no isolated fruit and wine party, with seductions in the offing; nor is it a picture to be admired for its formal qualities only. Giovanni Bellini painted it for Alfonso d’Este‘s camerino, or private study, in his castle at Ferrara, and Titian later reworked the sunny side of the background to harmonise it with “The Andrians” and “The Worship of Venus” (both belonging to the Prado in Madrid) and the National Gallery’s “Bacchus and Ariadne”. Reunited here, for the first time in 400 years, the four paintings echo one another; wonderfully busy, competing attractions.

Titian made sure the Duke’s eye would be distracted from the Bellini and flick immediately to the full frontal exposure of the nymph asleep, or aswoon, in “The Andrians”, while behind her the orgy threatens to get out of hand. Meanwhile, in “Bacchus and Ariadne”, Bacchus leaps from his chariot in a flurry of excitement and his followers march into view armed with cymbals, a tambourine and freshly butchered body parts. On the opposite wall “The Worship of Venus” completes the classical selection. At the foot of a Venus statue 100 babies with pink and blue wings squabble and scramble, their chubbiness Titian’s hymn to fecundity.

In the middle, a cherub pounces on a hare. In “The Virgin and Child with Saint Catherine and a Shepherd”, from the Louvre, the virgin soothes a white rabbit. Titian’s animals may have emblematic roles, but they are real. Even Actaeon, the hunter transformed by the goddess Diana into a stag, has credibility enough to be mauled by his dogs. In “The Death of Actaeon” he topples into the undergrowth like a sloshed Bacchanalian as ruthless Diana urges the hound to finish him off.

This late masterpiece, from the National Gallery, is one of many that remind us of so many others (“Diana and Actaeon” and “Diana and Callisto” in the National Gallery of Scotland, for instance) that are not in the exhibition. If anything, their absence emphasises that Titian’s greatness exceeds this selection, good though it is - and purposeful as it veers towards some of the most extraordinary pictures of all, rough, dense, uncompromising: the culmination of Titian’s long and successful working life.

The big question, preoccupying art historians but presenting no problem to anyone who works with paint, is the question of finish. Is “The Death of Actaeon” a completed picture? Or “The Flaying of Marsyas”, which hangs beside it - a horrific scene in which Orpheus plays his fiddle, King Midas looks on and Apollo, jealous of the musical accomplishment of satyr Marsyas, kneels beside his dangling body and gently separates skin from muscle, punishing him for his artistic pretension.

Twenty years ago, “The Flaying of Marsyas” was lent from Kromeriz in Czechoslovakia to another great exhibition, The Genius of Venice, at the Royal Academy. Since then, it has been cleaned and “restored”, and now looks curiously dead in patches. Even so, there is still the amazing sense of instincts and emotions sunk into the fabric of the canvas.

You can imagine the aged Titian looking again, and again, thinking of retouching, then deciding against it. The darkness and the solemnity, the grouping of killers and victim, onlookers and dogs, are composed into a momentous tableau of sombre method. Surely Michelangelo - “Last Judgment” Michelangelo - would have understood.