Robert Mapplethorpe's images went beyond his famous homoerotic photos: he also made portraits of glam 1980s New Yorkers. Why the different styles? Martin Caiger-Smith unzips the devilish conundrum that is Mapplethorpe.

Close your eyes and think of Robert Mapplethorpe. When I do, my persistent image encompasses flesh and limb, muscle and sinew, perfectly formed, straining against the bonds of leather; a mask, a knife, a zip, a whip, figures and forms trussed, pert or limp, the thrust of a stamen and the wilt of a petal, glamour and gore all masterfully posed, lit and framed, in velvet black, marble white and an infinity of sleek platinum tones.

'My approach to photographing a flower is not much different than photographing a cock, basically it's the same thing. It's about light and composition. It's the same vision.' This is one of Mapplethorpe's most famous utterances. It's characteristic of his provocative brand of honesty, and strikes at the core of our uncertainties as we look at his work and try to make sense of our own responses to it. It has been taken, mistakenly, to suggest that the artist drew no distinction between these two very different types of subject matter. But that's not quite what he's saying. Nor is it what we feel when we gaze in turn, in his pictures, at a stirring cock and a detumescent tulip. It would take the composure of a nun, and a dull one at that, to lock on first and foremost to a comparison of the photographs' abstract formal values. And who ever looked at one of his exquisite flower pieces and murmured to themselves 'Look at the size of that lily . . . '

Pity the hapless curator at the famous Mapplethorpe obscenity trial in Cincinatti in 1999, who felt driven, in his defence, to praise in one especially troublesome image the photograph's compositional values and in particular the strong 'centrality of the forearm'. What was in fact exercising the prosecution at the trial was the fact that the forearm in question (Helmut's) was buried deep up the back passage of his sexual partner, Brooks.

The trial had been triggered by a museum exhibition of Mapplethorpe's photographs, 'The Perfect Moment', which was then touring America. It followed an attack of moral outrage in Congress, spearheaded by the right-wing Senator Jesse Helms from North Carolina, on the National Endowment for the Arts, the important funding body which supported the offending museum. The whole affair gravely weakened the NEA, but through its blanket exposure confirmed Mapplethorpe's place in the history of art, of censorship, of late twentieth-century culture and (sexual) mores. He was, claimed the writer and philosopher Arthur Danto, 'the artist of his own time'. His own time, and place, was New York in the late 70s and 80s: the New York of poets and artists and socialites but mostly of sex, before AIDS, a 'night-biased world' of leather bars and S&M. Mapplethorpe, a young man in flight from a 'normal' suburban Catholic upbringing in Floral Park, Long Island, arrived in the early 60s and embraced it full on, trying in his art from the outset to reconcile its furthest points, to create work as arousing as he found pornography but also beautiful: to create 'smut that is also art'.

While Mapplethorpe now has his place in history, I guess the jury is still, and will remain for some time, if not out, certainly hung, on Mapplethorpe's stature as an artist. His photographs are often discussed in black and white, described in terms of polarities - darkness and light, erotic and chaste, dirty and pure, Catholic and Satanic, poised between the 80s 'moment' and the timeless classic - and his is a balancing act which cleverly plays and preys on the convolutions of artistic fashion and prejudice, the minefields and shifting territories of racial, sexual and gender politics, and on the perennial debates on censorship and individual liberties. Where we, individually, stand on it depends on where we draw the lines.

One of the valuable, if highly charged, aspects of an exhibition - as opposed to a book - is the public arena on which these debates are played out. To view Mapplethorpe's photographs in company, whether with friends or with strangers, is to realise how inalienably personal the judgements we make are as to what is good art or bad, what is erotic or not, what is acceptable or beyond the pale. Only more broadly, and over time, can any sort of consensus emerge. I was involved in the staging of a large Mapplethorpe exhibition, at the Hayward Gallery in London, ten years ago, and my recollection is that the response of those who visited the exhibition was so much more measured, equivocal and unpredictable than that of the media circus that surrounded it. A hush descended on the crowds as they entered. Outside, the whole event had all the ingredients of a Mapplethorpe jamboree: soul-searching discussions beforehand on what to show and what to leave out, speculation and leaked information, cries of foul play from Esther Rantzen, banner headlines, lawyers' recommendations and steers from the police (from whom I learned much about the routeless lands on the borders of 'obscenity', not least the wonderfully practical 'Mull of Kintyre' rule, whereby any image which portrayed a member standing prouder than that peninsula appears on the map could be deemed 'actionable' . . .) Critics and commentators decried him variously: he was not an artist, just a pornographer; or worse, from Brian Sewell, not an artist, just a photographer; a mere fashion photographer, said another; a 'derivative talent' another. Strong opinions were expressed by the black community and the gay press. His defenders were more eloquent, more nuanced: they had to be in order to be heard.

In general, the books Mapplethorpe produced organise his work in single themes: Nudes. Sex. Flowers. Portraits. His exhibitions tend to bring all strands together, asking us to move from black bellybutton to white longstem flower, from Sigourney Weaver in low-cut dress to Jim in Sausalito, leather-masked and crouching in a shaft of sun somewhere concrete, dirty and below ground. This can be a fascinating and productive journey, the ease of the glide a key factor, I am sure, in Mapplethorpe's success and renown. He shifts attention skilfully from one shifting territory to another, finding beauty in the base and vice versa.

So, flowers and cocks? I suspect Mapplethorpe would not be so widely remembered, 17 years after his early death, had he left behind only calla lilies and society portraits. Nor, I guess, might the SNGMA be showing him now, despite the fact that their selection of images appears to give a remarkably straight-laced and partial, almost dignified account of the artist's body of work. The whiff of scandal, even a distant one, will always excite attention. Is this show a genuine attempt to look beyond the controversy, to reassess Mapplethorpe's legacy, to select those images which will endure? Or a less laudable case of propriety, a kind of having your cake and not eating it? The hardcore scenes of sex and the raunchier nudes are almost completely excised from the picture.

What we are given, in the main, interspersed with flora, an occasional nude and the odd penis, are the portraits - some of himself in varying guises from vulnerable to demonic, of his great friend Patti Smith and the bodybuilder Lisa Lyon, but most, from the later 70s and 80s, of famous figures in or passing through New York. Artists: Warhol, Hockney, Lichtenstein, De Kooning, and Louise Bourgeois with a wry smile and her large bronze phallus tucked under her arm, the only genuinely funny picture in the show. Writers, stars, socialites: William Burroughs, Bruce Chatwin, Iggy Pop, Francesca Thyssen. No one more ordinary? 'I'm not interested in those . . . plain people,' Mapplethorpe is alleged to have said. There are no sharecroppers or bag ladies - the staple of American postwar photography - here. Only beauty; no gritty reality. And what of psychological insight? All his subjects seem to submit to the treatment, all appear somehow branded by him. 'His sitters,' wrote Bruce Chatwin quite acutely, 'whether celebrities or pick-ups - seem mesmerised not by the lens but by his presence.' One of Mapplethorpe's studio assistants said much the same: 'his sitters project their image of him onto the pictures.' There they stand and look at us, or at him, or into themselves, serene, composed and glacial: the very image of our image of a certain stratum of New York life in the 80s: chic, alluring, even slightly dangerous by association.

This is Mapplethorpe's true genius: the way he stays on the tightrope even today; the way he controls the image, convinces us of his ideal of beauty while straying further than we would think in search of it. Derivative? At times, yes, but in a blatant manner which manages to corral even the greatest icons of art history, the most familiar symbolism, into a vision that is also somehow unique. Mapplethorpe proves - like Dali - that an artist can be both meretricious and great. Does his perfect moment last? I think so. To speak eloquently of your time and to continue to do so to future generations is, after all, a timeless achievement in itself.

Martin Caiger-Smith was co-curator of a retrospective of Robert Mapplethorpe's work at the Hayward Gallery, London, in 1996.

Scottish National Gallery of Modern Art, Sat 29 Jul-Sun 5 Nov.