Self-preoccupied and revelatory, Francis Bacon faced Middle England with a sensibility it could barely tolerate.

Francis Bacon was sui generis. He didn't even have precursors in the Borgesian sense of the word - meaning precursors who were "created" by him, whose work is amended and endowed with previously unperceived meaning because of what it has inadvertently engendered. He does not cause us to scrutinise Velazquez in a new light because the gap between Bacon and Velazquez is chasmic. Bacon didn't steal the way great artists are supposed to. He took and joyrode and trashed. He was indifferent to the status of his sources: they might be works of the first magnitude, such as Velazquez's Pope Innocent X, or they might be medical illustrations. They were reduced to mere catalysts.

Nor did Bacon have successors. There was no school of Bacon. He fomented no fashion, suffered no disciples, occasioned no print other than his own, went against the grain. He was a figurative dissenter at the height of his powers during the hegemony of abstraction (which he regarded, scornfully, as mere pattern-making). He was just about inimitable.

This is a peculiar and rare situation, which affects Bacon's posthumous reputation just as it affected his reputation while alive. The history of painting and indeed of all creative endeavour is so lopsidedly biased towards -isms, movements, bogus groupings and distantly perceived alliances, that great originals are not so much overlooked as demonstratively sidelined. They have no place in the pageant of progress and continuous development. They inhabit culs-de-sac of their own making whence they are occasionally dragged to join a platoon of convenience, such as the School of London, which even by the extravagant standards of critical packaging is spectacularly spurious. Nabokov's dictum that there is only one school, the school of talent, is unexceptionable yet unheeded.

Bacon came from nowhere and led nowhere; indeed he might have elected to take such a course. His boasts of bibulous gregariousness and his aptitude for acquaintanceship hardly disguise his solitariness nor his concomitant lack of solidarity with other painters. He painted what he had to paint, what chose him. More wittingly, he painted what other painters didn't. He disliked the illustrative, the "literary" and the narrative as much as he did abstraction. It was the gap between these poles that he occupied.

Bacon was, however, part of a tradition of representational experiment and of painting as something more than drawing by other means. He was even perhaps the culmination of that tradition, the last great modern painter, a manipulator of marks and thence of sentience, of visceral and dorsal antennae. He addressed the core questions of human existence with a grotesque wit and a high seriousness that are entirely atypical of English practice.

Wilde's contention that "English art is a meaningless expression. One might just as well talk of English mathematics" is neat but wrong. English art - I know, there are exceptions - has tended towards the decoratively precise, the fastidious, modest, untroublingly pretty, above all towards the slight. It is not for nothing that the English medium was watercolour, with its unrivalled capacity for suggesting no colour.

Bacon did more than fling a pot of paint in the public's face. Technique, subject, sensibility: they may not have been deliberately gauged to offend but they most surely did offend and continue to offend to this day. A former editor of the New Statesman, the Sunday watercolourist Paul Johnson, is particularly sensitive to Bacon's buggering, blasphemous tours de force.

Middle England, that beige vacuum of dry niceness where all that's interesting is beneath the carpet, can cope with queers as long as they're camp and frivolous. But when they've got shit under their fingernails and cock cheese behind their ears and are piled on top of each other in sodomitical collisions which look like (and often are) war - well, that's not on.

And it can also cope with the nonobservance of religious rites - but to render the Virgin Mary as something between a lamprey's sucker and a toothy foetus is going too far, even if the woman did commit adultery with God.

That famous English tolerance has narrow limits. It is hardly surprising that Bacon is much more revered in France, a country less prone to squeamishness and more appreciative of (or more used to) emotional candour and self-revelation.

It is arguable that Bacon never painted anything but himself. When in La Nausee Jean-Paul Sartre describes an ink bottle's box as a rectilinear parallelepiped, he is not telling us much we don't know about such a box but he is, as Alain Robbe-Grillet has pointed out, telling us something about the kind of writer he is to use such a locution.

Bacon's portraits and self-portraits are perhaps the least successful part of his oeuvre. Not because they fail to achieve a likeness - despite the multiple mediations, the likeness is always there - but because they are Bacon's genre paintings, his most stylistically consistent works and the ones in which his propensity for self-cannibalisation is most damagingly evident. Among the photographs and prints he kept around him in his famously chaotic studio, the photographs which were his perennial props, were several postcards of his own work which he fed on with masturbatory indiscriminacy. This should not surprise us, for this was a painter whose off-white taches across finished canvases were expressions of emissive enthusiasm, of what the President of the United States calls baby-gravy.

Bacon's auto-plagiarism in areas other than portraiture had less deleterious consequences. Nonetheless the 1988 version (or near copy) of the great 1944 Crucifixion Triptych is the lesser work: it is slicker, more polished and it evinces a greater ease with paint. But it lacks the terrible rawness of the original. The introduction of more space around each figure renders the composition centripetal. The backgrounds are now elaborated, defined and bereft of the garish, grating poison orange of 1944.

It was the advent of this slickness and smoothness in the handling of paint that marked the onset of Bacon's long autumn - from the early 1970s onwards. But before that is 30 or so years' work whose intensity and compelling reinvention of the human body reward devoted scrutiny.

Of course Bacon represented humans as pieces of meat. Of course he created unforgettable tableaux of epic sordor. Of course he embarrassed both himself and his audience. But he did all this with such energy, such nihilistic glee, such earnest and such concentration that the work trespasses beyond the normal boundaries: not to say things that were previously unsaid, but to address senses of which we were formerly insensible and which politeness might bid us keep buried.