Day after day I find an excuse to be in Piccadilly and once there give up any attempt to stay out of the galleries at the Royal Academy.* Venetian art of the 16th century is running in one’s head like music, complete with its resounding orchestration. One abandons oneself to it. Yet it is not merely indulgence. One is drawn by the magnetism of an idea: the idea of art invented in Venice soon after 1500, at just the moment when this incomparable exhibition begins, an idea that one recognises, without reasoning how, as the modern idea.

In gallery after gallery at Burlington House the melodies of Venetian painting hang in the air. Anything that Giorgione had to do with seems to have been implicity musical – and he surely had to do with far more than he painted. The pictures at Venice and Dresden, the Tempest and the Venus, which are most certainly his work, are remembered whenever in the galleries the goddess lies nude and dreaming while turbid storm clouds roll up from the horizon to discharge their passion in the evening sky.

A dream was the natural medium of Venetian imagining. A print entitled A Dream of Raphael (apparently because the engraver later worked for him) is the opposite of rational Classicism. Baby monsters out of Bosch, just such monsters as infest Giorgione’s Sunset Landscape in the National Gallery, crawl from a canal into a catastrophic townscape, there to haunt two slumbering Giorgionesque nudes. The flaming town is a darkened arena for an extraordinary mastery of graphic tone. Imaginings like this are the common context for the musical harmonies of Venetian art.

The visual music is an extension of consciousness which is in both senses as fantastic as the widening frontiers of thought in Tuscany and the Rhineland. It was akin to the exploratory resource of Leonardo, who was in Venice twice in the first years of the century, perhaps showing Giorgione a roundness as impalpable as smoke and teaching Sebastiano how to model an eerily palpable thigh under St Louis’s robe. The music was there already. It flowed through the Alpine passes with the Northern travellers. Nightfall in the countryside above Bergamo was the setting in which Cariani’s Lutenist – like an offspring of the couple in Giorgione’s Tempesta, brought up to just their suspenseful mood – thoughtfully attended to his melody, while a shepherd with a pipe (sheep were herded to the flute in Venetian painting) walked though a darkening farmyard towards the mountains. The Pastoral Concert must have been inspired by Giorgione, even though Titian painted the picture (in the Louvre). In mid-century (and often in this exhibition) the concert reassembles to celebrate erotic delight and fate. In one print the band is conducted by an écorché figure and a skeleton, bodies flayed for art; the subject is known as the ‘Aviary of Death’.

At the very start, in the Large South Room, which at other exhibitions one is inclined to miss, yet now cannot stay away from, one meets one of the great puzzles among the pictures that Giorgione had to do with, the Judgment of Solomon, unseen for a generation in a country house in Dorset. It is found to be a highly intellectual picture, by someone who was seriously interested in the receding depth of a basilica and the justice that was dispensed in it, with a stepped perspective as exact and spacious as in Giorgione’s Castelfranco altar-piece. So much in the picture is so personal that it should surely be possible to recognise its painter. The taut, diagonal creases in the judge’s cloak, for example, which are so well conceived to agree with the lunging and retiring diagonals of the action: they inaugurate the zigzag folds which catch the light in Venetian painting, not only modelling shape but proposing pictorial directions, prompting the significant gestures of the brush, which are Venetian painting. It is clear that Sebastiano del Piombo, lately the favoured candidate for the honour of having painted the Judgment of Solomon, which, unfinished and damaged as it is, is still an indubitable masterpiece, is unlikely to be the right one. His organ shutters hanging opposite disqualify him. The mothers, between whom Solomon must judge, are clearly, coolly drawn, one of them in profil perdu: they are quite evidently not Sebastiano’s monumentally sultry sybils.

What great draughtsmen these painterly colourists were! Perhaps Giorgione drew with such functional clarity precisely because he insisted on launching straight into colour. The series of hues strung out across the Judgment now emerges as one of the most serenely resolved colour-scales in art. On the left, the soldier in silver and red is the brother of the soldier in the same square cap who has been severed from but now restored to the so-called ‘Adultress’ from Glasgow. The women beside him in moss-green and mauve lead to the constable in apricot and silver; the boy behind has a relative, similarly breaking out of the penumbra, in the Pastoral Concert. The judge in slate-grey and blue, with his pronounced, diagonal decisiveness, is the pivot of the whole picture, against the mauve pink cloth of honour that hangs with such exact chromatic justice against the depth of space. Then the sequence continues, with its consistent freshness. The bearded sage from among the Vienna Philosophers, with his eloquently drawn hands, is softly, gently modelled, like the old man in the Glasgow picture and in the San Rocco Bearing of the Cross – modelled with transparent roundness as only one painter did it; he is in cinnabar and indigo. The true mother’s goodness is epitomised (and aligned with Solomon) by a chord of blue white and emerald. The agonised courtier who understands takes up the indigo and apricot. The sequence leads at last to the executioner in whose dusky colour and diagonal thrust the picture is concentrated and concluded.

We are present at the exact moment when the local colour of the 15th century changes into the tonal colour of a new age through a system of finely intelligent and intelligible modulations that allow the eye to exchange the scales of value and hue – a system of tuned affinity and compromise achieved without apparent impurity or loss, analogous to equal temperament in the keyboard to come. Now that the Judgment hangs beside the Glasgow picture (in which I persist in seeing Susanna and the young Daniel), both reveal their beauty as never before. It is a sign that we have pictures in their right context. Can we really doubt who these two masterpieces were predominantly due to? Looking at the Judgment of Solomon, we can imagine the radiance and clarity of Giorgione’s vanished frescoes on the outside of the German Warehouse (now the post-office) at the Rialto. In the Glasgow picture the obsession with light on crumpled drapery is found to be the special property of the flickering brush, which points – and how pointedly – towards the greatest Venetian painting of the century.

Veronese, whom we honour for limpid, daylit iridescence, was equally lucid and gracefully specific as a draughtsman. We can judge his feeling subtlety when he is modelling a head with colour – Procris, for example, mourned by the careless and repentant Cephalus, in a picture (borrowed from Strasbourg) which balances with tragic gravity the buoyant blue and orange of its pendant (from Madrid), the famous Venus and Adonis, in which consummated love is lazily triumphant. Triumphantly Venus and Adonis are posted up all over London, and for once a poster does something useful, apart from attracting us to an irresistible exhibition. If you wonder why the Prado picture has never looked so good, you can thank the poster for restoring its concentration by silently omitting the equivalent of half a metre along the top, which is an unlucky addition.

Under the leadership of Titian, Venetian painting turned in a direction that what we know of Giorgione barely suggests. Form was generated by colour as it had never been before. Colour marked the quality of things that were luminous of their inward nature – and still are when a painter has the constancy to achieve the steady, sensuous state that can be wrought in the ductile stuff of paint. In the 12-year-old Fanuccio Farnese’s tunic, painted by Titian in 1542, crimson breeds a golden lustre which condenses into glistening embroidery and materialises, bulging, in the buttons. ‘Finally splendour was added, which is something quite different from light.’ Painters who read Pliny would eagerly have sought that splendour. Movements of the brush are direct and purposeful, like the impulsive gestures that spread the crumbling incandescence along the zigzag folds of drapery, moulded by the wearer or billowed by the wind and reflecting, in its directional impetus and chromatic magnificence, both the outward glamour and inward emotion of painting. The two culminated together in the pictures that flowed from Titian through the third quarter of the century.

A richness of material and imagining that was intrinsic to oil paint is the dominant theme of the exhibition, and its dominance was due to Titian. Lotto, the most individual talent that Venice bred, was only himself in Bergamo. One can see that the sensational disproportions in his manner, or the amusing fear and ferocity inspired in the Virgin’s household cat by the Annunciation, would have been excluded by the convention of grandeur that Titian promoted in Venice. Conversely, young men from Bergamo, like Cariani and Palma, who sought their fortunes in the capital made more use of the Giorgionesque example than any painter native to the city. Paris Bordone from Treviso must have been schooled in conformity. He made the Mystic Marriage of St Catherine into the most courtly and fleshly of sacred conversations, surprisingly set in a landscape almost unaltered from the Pastoral Concert. In later life Bordone made a corner in florid realisations of mythology with abundant rosebuds and shot silk, which were the height of poetic luxury. Wherever the spell of Venetian power and wealth was felt, Perseus armed and Venus crowned would have been thought of as Bordone painted them. The sumptuous figurative resource of the Italy and the Antiquity imagined by Shakespeare was not far away.

Across Gallery III the accumulating coolness and warmth of Veronese and Titian face one another with staggering effect, broken only by a splendidly energetic full-length of a cavalry commander ascribed to a certain Brusasorci, which, if it was really by him, would make him at a stroke one of one’s favourite painters. The armour is striped in shining bands, which echo the contours with arresting kinetic effect: as one moves gingerly past, the commander strides too. His family thought that he was painted by Veronese and such a presence was hardly within the reach of lesser men.

At the far end of the gallery Lombard realism takes over, and sustains the comparison with Venice better than anyone could guess. The catalogue describes Brescia (where the striped armour was made) as ‘the most avant-garde artistic centre in the Veneto’: it is delightful when historians permit one another such liberties, and makes one wonder what the expression can mean. Certainly the cool objectivity of Brescia anticipates a common temper of 17th-century painting, just as the other kind of Brescian picture, the hallucinating visionary grey in which Moretto painted a dramatic Ecce Homo, offers a phantom foretaste of Zurbaran.

It is not at all clear what such unhistorical observations are really about. Painters in the hill-towns of the Terraferma certainly recorded human likeness with a social awareness that was not encouraged by the stately nobility of the capital. Lombard sobriety had by comparison a hard-bitten existential reference that must have been apparent and attractive. Northern travellers stopped at Brescia and there are drawings of Lombard pictures in Dutch sketchbooks. Across the next hundred years there are delightful coincidences: Cariani has a splendid brothel scene (always mistaken for a family portrait) which is like a prototype for Vermeer’s Procuress, just as his House of Martha at Edinburgh resembles a Moretto. One begins to doubt whether anything in the image stock is wholly unknown to any participant and presently ex-historians fall to wondering about a collective unconscious. The only undoubted fact is that Lombard painters maintained the empirical method that was one potentiality of the Giorgionesque innovation, and there in Lombardy it remained until a young man with a pathological depression came along to lay hold of it with a fury that set disegno at naught. At the end of the century a good judge could see nothing in Caravaggio’s St Matthew pictures but ‘the thought of Giorgione’. The mysterious avant-gardism and the impression of a Lombard current flowing strongly towards the future are due to the fact that Venice and the Veneto offered to European painting the 17th-century revolution and its 15th-century source, both at once.

In the centre of Gallery III, dominating the unparalleled assembly of Titian’s last pictures, hangs the Flaying of Marsyas, which has not been exhibited in the West since 1673, when a bishop in Bohemia won it in a raffle. All these months – it is not too much to say – London has been half under the spell of this masterpiece, in which the tragic sense that overtook Titian’s poesie in his seventies reached its cruel and solemn extreme. At most hours on most days there is a knot of visitors riveted and fairly perplexed in front of it. It is a controversial picture, not only in the way that late canvases by Titian are apt to be. Some of those who know the artist best doubt whether in basing so much of our view, and not of Titian only, on canvases that were left unfinished, we do not miss the coherence and deliberation of his purpose. That doubt can be set at rest: the Marsyas is as consistently and in detail as finely wrought as pictures that Titian valued and sold – and the canvas was signed. The serious doubt about the Marsyas is felt just because the story is so explicity told.

In the autumnal woods a satyr is tied head down to a tree, from which his pipes are hanging, to suffer the punishment for challenging Apollo to a musical competition, which he could not by definition win against an antagonist who was none other than the inventor of music. With a rustic assistant (wearing a Phrygian cap) Apollo, identified by golden hair and a wreath of laurels from Parnassus, with almost loving attention is skinning him. A puppy laps up the blood that is shed and a hound is restrained by a child of the woodland people; another satyr comes with a bucket of water, and Midas, the judge, broods sadly on his failure to defend the mortal from the god. Behind, as god-like as anyone, a musician in crimson plays a lira da braccio with feeling. The controversy is an old one. Lessing thought Ovid’s account disgusting and those who have difficulty with the picture, in which Titian followed Ovid closely, are echoing a painter of the Enlightenment who told Goethe that Raphael himself would have done better to avoid it. At the Academy people still ask, and on the radio well-meaning critics debate, how it is possible that a horribly painful subject should be the occasion of beauty or greatness in art.

As always with painting, we must look at what the picture is about. Certainly it is about the fate of a victim – which is to say, it is about identity and suffering. Ovid tells that Marsyas cried out: ‘Why are you stripping me from myself?’ It is about the presence in a body. The shaggy haunches are spreadeagled like a Vitruvian figure upside down, as if to demonstrate the satyr’s noble proportions. Titian models his body to show how ‘his nerves were exposed unprotected, his veins pulsed with no skin to protect them. It was possible to count his throbbing organs and the chambers of the lungs clearly visible within his breast.’ Titian rubbed the golden colour into tender opalescence over the modelling of the chest. We notice what our own art has equipped us to see; Francis Bacon, who sometimes compares the exposure of a human subject with a carcass split in half, paints what is inherently vulnerable in a body. Ovid tells that ‘the fertile earth grew wet with tears ... received the falling drops into itself and drank them into its deepest veins.’ Marsyas was indeed poured out like water (the account is curiously like the psalmist’s prophecy). Not even the dogs that compassed him are missing from the picture. The stricken judge is enough to show that the painter knew grief, as Ovid did: ‘the woodland gods, the fauns who haunt the countryside mourned for him; his brother satyrs too, and ... all who pasture woolly sheep or horned cattle in these mountains.’

It is not easy for us that a picture which is about the punishment of arrogance should also be seriously to do with music. It was on behalf of order and the laws of harmonious proportion, which sound in the music of strings, that Apollo claimed victory over the chaotic and impulsive sound of the pipes. The disproportion was not musical only: Minerva, who invented the pipes, threw them away when she saw in the water that blowing them distended her cheeks. The punishment to which arrogance was submitted was itself a Dionysian rite, a purification that peeled off the ugliness of the outward man. Apollo’s ideal, inscribed on his temple (from which Socrates, outwardly a satyr, adopted it), was self-knowledge, and Dante began the Paradiso with a prayer to him: ‘Enter my breast and infuse me with your spirit, as you did when you tore Marsyas from the covering of his limbs.’ Plato said that the strains of the pipes, which he excluded from the Republic, indicated those who were in need of the Gods. With Titian’s Marsyas, so far from suffering cruelty, it appears that a need has been fulfilled. His eyes have a rapt and trance-like gleam. He is spellbound, transported by the rite.

One follows the story across the canvas, watching the smouldering colour, fanned or flicked into incandescence, then dwindling and swept broadly into shadow. Then one turns back to the magnetic player of the lira da braccio, as if to listen; very likely he is accompanying himself in song. Philipp Fehl, whose study illumines this picture, has connected him with the remaining figure in Ovid’s account. He is a musician called Olympus, the pupil of Marsyas who loved him, and the mourner to whom Apollo gave the body. Olympus was converted by Apollo’s music to harmony and to the lyre; he wrote a hymn to Apollo; perhaps he is singing it. He remained ‘dear to Marsyas even then’. The melody which he contributes to the picture is the reconciliation of opposing principles, and it reconciles us to the theme.

It is quite a moving moment in Piccadilly in which we understand even a little of the mystery. There has been nothing quite equal to it in Gallery III since Mussolini unpardonably shipped the Birth of Venus to London in 1929. I saw it as a schoolboy; the sight of it over the heads of the crowd, and the awareness of the mystery, remain almost untouched by familiarity since. Tuscan painting has more in common with the Venetian experience than one would think. Bordone’s St Catherine, for example, in her Giorgionesque landscape, advances with just the tripping step of Botticelli’s Hour of Spring. Perhaps the painters were both imbued with the same lines of Ovid. But this year one attends to what is unique to Venice, and there is much more of it in this exhibition, more enlightenment, than one can possibly tell. The roomful of pictures by Jacopo Bassano produces the kind of knowledge that only a one-man show by a very good painter ever yields, the all-embracing knowledge of a personal context which adds to the meaning of every separate picture. It is a circumstance of this incomparably personal art of the West with which it remains for criticism and aesthetics fully to deal. Bassano, whom we are taught to respect for his bucolic inelegance and voracious country appetite, turns out to be a refined, discriminating intellect apparently of the opposite kind. Not instead – as well.

Who could have guessed, when the story of the Tintoretto family’s Paradise, in the Doge’s Palace, seemed complete and fairly wearisome, that a glorious modello (now at Lugano) was in store, as marvellously virile and wholly autograph as anything that ‘the strongest of the Venetians’ painted in his Herculean old age? Almost the only great master in Venice who was a natural-born Venetian, there was always within his power a spectral (yet bodily, muscularly corded) transformation-scene, which would contribute still more to the specifically Venetian discovery. Venice made the West aware of its least Platonic intuition: that it is the actual physical material that comes to life in Western art. The material of Western art has nothing to do with impurity: it is the stuff of imaginative embodiment. I should regret the immense success of the show if the genius of Venice were known simply as a notoriously good exhibition. In fact it is a dimension of the human potential which one is completed in, or impoverished without.