Early one morning in Philadelphia, I was walking over the South Street Bridge on my way to work. My path was a narrow walkway between steel-bolted panels on the traffic side and a lacework of wrought iron on the other. The Schuylkill River shimmered through the iron curlicues in oily ripples, and at regular intervals, the lacework opened into bays overlooking the water. But I was not looking at the river. My eyes were fixed on the steel panels on the opposite side of the road, where someone had spray-painted a poem over half the length of the bridge. It was about lost people looking for each other by the river, but because the poet had been able to fit only a word or two on each panel, the poem unfolded very slowly. I kept craning my neck and squinting to see how each line would end, wishing I could walk as fast as the poem demanded. Suddenly, out of the corner of my eye, I saw a pair of legs in a bay ten seconds in front of me. I exclaimed involuntarily, for what I saw was not only legs, bent at the knees, lying on the ground in weathered trousers – perhaps green – but a dark penis, too, and a hand idly rubbing it up and down. The body and head were hidden in the bay. I was over halfway across the bridge; it would have been very inconvenient to turn back. I could not climb over the steel panels either, since I would have landed in traffic. Besides, that seemed an inappropriately desperate response to the situation. If he did not mind lying there like that, I reasoned, he would not mind my walking by. And so, in the nanosecond in which such decisions are made, I resolved to keep going, and before I exhaled again, I was past him.

Later that day, I described the scene on the bridge to a group of friends, and after much head-shaking all round, someone told a companion story. This man and his wife, having run off to Paris long ago, deeply in love, took a ride on a bateau-mouche. A guide was pointing out the glories of the city over a loudspeaker: ‘Voilà Notre Dame. Voilà l’Ecole des Beaux Arts.’ The boat passed under a bridge, and suddenly everyone stared upward. A man was hanging from the underside of the bridge by one hand, and masturbating with the other. ‘Voilà un homme formidable,’ the guide said.

In America, what with political correctness and other forms of institutionalised earnestness, the figure of the outsider seldom evokes this sort of amusement. Perhaps the last such ‘homme formidable’ was the artist, Jean-Michel Basquiat, whose work was shown in the spring at the Serpentine Gallery. A black-Hispanic graffiti artist, Basquiat acted the outsider with brio. His career opened in 1981; he became famous overnight; and died of an overdose in 1988 at the age of 27. In between, he painted wearing an Armani suit, collaborated with Andy Warhol, played bells and triangles in a New Wave band and rode in limos for the hell of it, or maybe because New York cab-drivers do not pick up black men. He also completed over five hundred canvases and many drawings.

Initially celebrated as America’s first black art star, Basquiat is now the subject of more ‘serious’ assessment. A full-scale retrospective at the Whitney in 1992 was met with scepticism: it seemed too soon to propose this media-smart graffiti artist for inclusion in the American canon. At the Serpentine, he was treated so solemnly that every hip-hop rip-off became either an act of aesthetic homage or an expression of black pride. Either way, the problem with Basquiat’s reception is largely a function of the Eighties. He died just after the crash of 1987, and the sobering correction in prices and values that followed seemed to indict the painter together with the decade. His major museum-quality canvases have fallen in price from the $400,000s of the mid-Eighties to the $200,000s and $300,000s fetched today when they sell at all. His father has pulled the estate back from Basquiat’s dealer, Robert Miller, hoping to make more profit from private sales. And this even after the Whitney retrospective, a recent show at Galérie Enrico Navarra in Paris and a movie in progress by the artist Julian Schnabel, with David Bowie as Warhol and Jeffrey Wright as Basquiat.

The curator Robert Storr has coined the best term for Basquiat’s work: ‘Eye Rap’. Basquiat was a punster in the tradition of Warhol and Nauman, painting on refrigerators because he was cool, making graffiti art to leave his mark. He signed himself on walls and canvases as SAMO to signify ‘same old same old’ or ‘same old shit’, according to his friend, Fab 5 Freddie. For this black-Hispanic rapster, racism was a routine throwaway among the rest of the urban garbage. When he scrawled TAR on his canvas-walls he was also writing ART. In his art-graffito SAXMO, he linked SAMO to Satchmo, but also, as Klaus Kertess has noted, to Little Black Sambo; Basquiat was signing himself as culture-hero and Uncle Tom in the same word. Wilfully obscure and obtuse, no sooner did he write himself in than he erased himself. And the erasure just made him more visible. ‘I cross out words so you will see them more; the fact that they are obscured makes you want to read them.’ One of Basquiat’s greatest achievements was to awaken this desire to read him, while all the time letting us know that the desire was doomed.

In this game of here/not here, Basquiat revelled in language and signs. He found a handbook of hobo symbols – a secret language for social outsiders – and added them to his canvases. He copied out book indexes with entries misspelt or crossed out, as if to match in words his travesties of anatomical drawings. The meticulous lists of historical dates and names always break down, for in his work cultural transmission is in a state of crisis. Trademarks, brand names, the copyright sign, the notary public’s seal – Basquiat focused on words that create ownership, limit usage, replace the object they name. By painting them, he appropriated them for his own needs and, in the process, shifted their meaning. He is part of the symbol-politics of the Eighties, which acted as if ‘signifying on’ the white and the rich would redistribute power and wealth. It did not, but the signifiers did seem to have more fun.

A cultural crossroads, Basquiat had Haitian and Puerto-Rican parents and grew up in Brooklyn in comfortably middle-class circumstances, speaking English. Living with his father in Puerto Rico for a year and a half, he learned Spanish and later became fascinated with Santería and other Afro-Caribbean religions. He wrote graffiti on city buildings as a teenager, but took care to do so on SoHo galleries on opening nights, when artists and dealers were likely to be looking. At home in New York slums and at society parties, he told Henry Geldzahler that his subject-matter was ‘royalty, heroism and the streets’. He saw Andy Warhol, with his platinum wig, as the closest thing he had to a father, and modelled his own hair-do on the Apollo Belvedere. His art is literally a picture of New York, a pastiche of graffiti words and drawings, of trademarks for products and the garbage they become. His figures are cityscapes, too – their brains a transcription of graffitoed walls (Untitled Skull, 1981) covered with scars and fences and signs. Basquiat’s work is about race and class and the ludicrous ironies they write across New York City.

It is also about art, however, and his range and parodic invention are impressive. In Boy and Dog in a Johnnypump (1982) or any of the savage figure paintings, Basquiat daringly acts the primitive, yet at the same time, his figures’ dangerous teeth are unmistakably rendered as a Modernist grid. His scale, brushwork and colour are those of Abstract Expressionism; his figuration and lettering suggest Pop and graffiti. Basquiat’s influences range from Warhol and Jasper Johns to Cy Twombly, Leonardo and the comics. William Burroughs was his favourite writer. His use of words encompasses both Art Brut naivety and the semiotic sophistication of concrete poetry. In his visual-verbal writing, Basquiat gives new meaning to the Futurist phrase parole in libertà.

With this bottomless capacity for imitation and appropriation, Basquiat became the first Afro-Hispanic artist to enter the mainstream. As such, he ended up in the position of cultural translator to the enemy. When white artists have tried to deal with the culture of the Other – James Agee, for instance, or the writers and photographers of the Works Projects Administration in the Thirties – the results have been patronising, guilt-ridden and absurdly idolatrous. Basquiat, working from within, can act the primitive and gross out the politically-correct establishment. For all the hagiography of black boxers and jazz musicians and the citing of racist and capitalist evils in his works, he is an irreverent jokester, a master of the put-on. Leonard da Vinci’s Greatest Hits (1982) is Basquiat’s homage to the Italian master, and at the same time a spoof of Leonardo’s anatomical drawings. A botched rendering of a foot is labelled BAD FOOT; a heroic nude holds a machinegun; the word TORSO stands in for what it signifies, above Kilroy genitalia overrun by a ladder|traintrack|scar.

How can the celebrity outsider maintain a sense of his identity, or painterly authority, when he is his own subject-matter and his audience sees that subject-matter as ‘other’, less than ‘us’? Basquiat’s solutions to this dilemma are often brilliant. In the triptych Zydeco (1984), for example, a cinematographer in profile looks through the lens of his movie camera. In a Cubist pun, however, the head also reads as a frontal view, with the eyes looking outward at us. The filmmaker keeps one eye on his viewfinder and two on his viewers, and what he shoots (or projects – it is hard to tell which) is a stick figure painted on a canvas whose head is a rectangle with the word SUBJECT on it. Basquiat’s subject is subjecthood, or language, or himself as a shifting word. Above the ‘canvas’ is the copyrighted warning, DON’T LOOK IN THE CAMERA© – the apostrophe (the sign of elision) made out of the tape or peg that holds up the canvas. With the copyright sign after it, the warning is itself a quotation, ‘owned’, presumably, by Basquiat himself. But its message is totally ambiguous: don’t turn us into subjects; if you must be a subject, don’t let the camera see your eyes; or maybe, don’t look into the mechanism of my art.

In a culture created by other people, Basquiat bends their signs and symbols to his own use and plays on their stereotypes, while he himself, thanks to the endless shifting of language, remains multiple, elusive and at the same time as direct and in-your-face as the rappers he played with. He hangs from the bridge as the bateau-mouche passes, and lights up our faces with a complicated smile. However uneasy it might make us feel, the response he elicits is finally more salutary than the guilt and earnestness that so many artists since Basquiat have inspired.