The key to Caravaggio

"It cannot be too strongly stressed," writes one contributor, "that we still lack a key work for a proper understanding of Caravaggio's art in his very last years of life," by which he means a Sicilian painting stolen by the Mafia in 1969, transported to the mainland and last seen in the keeping of the Mafia priest of a village south of Naples. So there we have it - no firm conclusion can be reached without this perhaps lost Nativity, and I agree, if my fading recollections of seeing it in 1965 are in any way reliable.

But when the same writer goes on to say that The Crucifixion of St Andrew "has little in common with Caravaggio's last works" yet nevertheless places the picture in the first Neapolitan period, one begins to wonder whether art historians can agree even on the meaning of such simple words as late, last and final applied to a period of fewer than four years - Caravaggio died in July 1610.

Another writer on another picture tells us that "the question of the date has caused much ink to flow". So it has, in this and almost every other case, to the extent where the visitor may well feel bound to ask whether any of it matters; it does, but not quite so overwhelmingly.

My own, perhaps too simple observation is that when art historians sink their teeth into a date they are like mongrels fighting over a sausage - they lose all sense of quality. What, above all, is clear in this exhibition is that Caravaggio in his final years was very variable in quality, both within and between pictures, and that some of them, though undoubtedly authentic, are poor things compared with the masterpieces that he consistently produced in Rome.

What brought about the change? Why did Caravaggio leave Rome? Born in 1571 (1573 according to books written before the recent discovery of a definite record), Michelangelo Merisi adopted the name of the small town of his birth, Caravaggio, some 25 miles east of Milan.

In Rome, at 21 or 22, he began to build a reputation as a painter, first of androgynous boys to tease the loins of homosexual cardinals, and then of shadowed shaft-lit dramas of betrayal, violent martyrdom and sordid death, the down-atheel urban peasants whom he used as models an insult to the saints as
whom they posed.

The claustrophobic savagery and sadism of his images were heightened reflections of his life as bravo and braggart, brawler and swordsman and as the lover of sensual boys at least as much as women, yet he was a man who could fatally knife one in the groin over a disputed game of racquets.

This random act of manslaughter in 1606 was the immediate cause of his flight to Naples. At 35 or so he was notorious as a man whose brain was menacingly addled; from 1600 on he is in the records of the police linked with acts of mayhem, for throwing stones at his landlady's windows, chucking a dish of artichokes at a cheeky waiter's head, assaulting a notary who had propositioned his pet prostitute on the Piazza Navona beat.

He seems to have sought out and provoked opportunities for irrational violence and there was, perhaps, something of the torturer and executioner in Caravaggio himself, his unpredictable behaviour and twisted mind reflected in his pictures through the violence of the low-life louts and syphilitics who torment Christ, crucify apostles and behead the Baptist.

Caravaggio's Jacobean sense of horror for horror's sake was an early contribution to the 17th-century cult of the terrible so evident in art and theatre, but in Rome it had been leavened by a profound and immediate response to physical beauty in both boys and women, and even, perhaps, in his discovery of a less obvious and more melancholy and spiritual beauty in the sagging flesh and wrinkled faces of the old.

In Naples there was far less leavening; his eye for beauty so physical that the loins respond was dimmed, the sensual youths disappeared, the women were reduced to formula; emotion and expression became the preserve of the careworn old.

In what is almost certainly the first major painting of the first Naples period, wholly independent of obligations established in Rome, The Seven Works of Mercy, some of his sensual beauties remain - both boys and women - but this key picture in the examination and construction of Caravaggio's late work, the turning point between the great heights reached in Rome and the falling away that followed in the south, is not included in the exhibition.

Nor is The Beheading of the Baptist, painted for the cathedral in Malta, without argument the greatest, most perfect, least hurried of all the late works, a composition of wonderful proportion, interval and cold logic and a clue to what he could achieve in a brief period when he was not in fear and his mind at peace. Without these two extraordinary paintings, the rocks on which all argument should be based, their quality sublime, the exhibition is almost pointless.

It is, nevertheless, a remarkable achievement to have brought to London paintings as large and impressive as The Raising of Lazarus and The Adoration of the Shepherds from Messina, as important for their violent imagery as The Crucifixion of St Andrew from Cleveland and The Flagellation from Naples.

The comparison between the National Gallery's popular Supper at Emmaus of 1601 and the revised treatment executed in a picture borrowed from Milan, with which the exhibition begins, is more illuminating than mere words could ever be, but in the light of The Seven Works of Mercy I question the date given to the Milan picture as the summer of 1606 and thus painted before Caravaggio reached Naples.

It must surely, on grounds of style and handling, be later by quite a margin, all Roman references gone, its physical types taken, not from live models, but from an established repertoire much used.

With these and other, lesser, pictures, so dramatically presented that the visitor must feel that he has unwittingly intruded upon a theatre's stage, there is enough to demonstrate the disintegration of Caravaggio's abilities and perhaps his sanity.

His command of largescale composition broke down and his smaller pictures verged on unconvincing awkwardness, character became caricature, narrative was reduced to strip cartoon simplicity and, perhaps most obviously, a painter for whom hands had long been the most expressive details of despair, alarm, accusation, distress, tenderness and benediction became, in the very late paintings, as informative and anatomically convincing as puddings boiled in socks.

No one should risk disappointment by going to this exhibition unprepared. I commend two straightforward books: The Complete Paintings of Caravaggio, by Michael Kitson, 1967, still to be found second-hand for little money, and Caravaggio, by John Gash, 1980 ( reissued in paperback in 1998); they may not look much but their texts and notes on illustrations are admirable.

A third book, just published by Pallas Athene at £6.99, The Lives of Caravaggio by Mancini, Baglione and Bellori, gives us in English the three 17th-century sources on which modern scholarship depends; good and useful, it is, alas, introduced by Helen Langdon, whose own biography of the artist should be avoided.

The exhibition catalogue is for experts only; confusing in layout, it does not even offer a chronology and, at £25 for a middling paperback, is absurdly expensive; in it the Director of the National Gallery pays "special tribute to ... Michael Kitson" and, by implication, his slim book - buy that instead.

Caravaggio: The Final Years is at the National Gallery, Trafalgar Square, WC2, until 22 May. Daily 10am-5.15pm (Wednesdays 10am-8.15pm). Tickets £7.50.