Where should we draw the line between what is obscene and what is culturally enriching? More books have been written on the subject than perhaps any other in art. But still it continues to haunt us.

The latest author to throw her hat in the ring is Cambridge academic Alyce Mahon, who, as the publicity so coyly puts it, is "a woman". This is not really so revolutionary. Some of the most stimulating writing on the subject produced in the past 30 years has been by women, notably Andrea Dworkin. Where Mahon supposedly differs from her predecessors is that she sets out to address her topic without a specifically political agenda.

Far from adopting the classic feminist critique, she seems to be attempting to be objective. This, of course, is no less than we would expect of a 21st-century art historian, and her book, despite being fundamentally flawed, makes for a read that is pacy and questioning.

Mahon restricts herself to the period from the mid-19th century to the present day, and having assured us that she is not attached to any particular pressure group, she then proceeds to construct her own. She begins by attempting to distinguish between the erotic and the pornographic. Erotic art, we are told, does not exploit sexual imagery for the purpose of titillation. There is always within it another intent - "a shocking means to express social, religious and political criticism or defy bourgeois taste". On the other hand, "Pornography's sole intent is to stimulate sexually; it is an aid to sex or masturbation."

There is some truth here, yet while eroticism might not always intend to arouse, it is surely too much of a generalisation to suggest that it always has a political motive. Mahon's definition of eroticism seems simply too narrow. By imprisoning herself in this premise, the author in a sense refutes her own objectivity before she begins, though not so much that it is not worth persevering with this thoughtful book. Do not allow yourself either to be put off by the author's tendency to pepper her generally readable prose with often too weighty comments disguised as throwaway lines - chief among these in the introduction being the "fact" that Immanuel Kant was David Hume's "more influential contemporary", and, indeed, her reference to Hume as an "English" philosopher.

MAHON BEGINS her excursion with an examination of the depiction of the nude, and eventually reaches the now well-seasoned imagery of the likes of Mapplethorpe and Della Grace, which still never fails to provide today's tabloid press with crowd-pulling headlines. It's a bumpy ride, but hugely entertaining and provocative.

For example, exploring the idea of the female nude as sex object - from Goya's Maja to Ingres' oriental harems, she rightly interprets the latter as having their erotic roots in the fact that all such supine women are under the control of one man. This surely makes such work, by her own definition, pornographic, in that it is related to a violent, empowering, specifically male image of sex. The male nude, she goes on to say, is traditionally perceived rather differently - in a homoerotic context.

I cannot help thinking, though, that we eventually begin to lose sight of the original premise of what makes eroticism and pornography essentially different. Looking, in particular, at the Surrealists' preoccupation with de Sade, it is surely undeniable that Masson's hardcore drawings and Dali's subtly expressed preoccupations with sodomy and other taboos were specifically created to stimulate and arouse, rather than with any political end in mind.

We travel in a similarly random fashion through the politics of the 1960s and 1970s, stopping to admire Judy Chicago and Carolee Schneeman's explicit auto-sexual performances. Mahon then tackles the feminist debate of the 1980s and its appropriation of the male porn industry, examining two artists in particular. Annie Sprinkle was the ex-prostitute who, in the 1990s, famously staged a performance in which she used a sex aid on stage and then invited the audience to examine her in intimate detail. Jeff Koons, equally infamously, produced a series of photographs of himself copulating with his wife, an Italian porn star.

Both artists proclaimed their work was intended as a pastiche of, or a direct attack on, the porn industry. What Mahon fails to answer is how you prevent the apparently good intentions of artists such as Sprinkle or Koons being subverted, and the art itself being used as pornography. The answer is you can't. Koons might intend his Made In Heaven photographs and sculpture to be a critique of porn, but inevitably they end up as its instrument - conveniently packaged for middle-class consumption as 'art'. And, of course, this realisation is all part of Koons's knowingness.

Finally, and inevitably, we come to the Chapman brothers and their disturbing zygotes, Matthew Barney's bizarre Cremaster video, and Tracey Emin's bed.

It is a necessarily selective survey, but there are significant omissions, not least Gilbert & George, as well as the erotic potential of text-based art, and that still under-rated Surrealist transsexual, Pierre Molinier.

The major problem with this and any book on art and eroticism is that, inevitably, no matter how objective they want to be, the writers are doomed to be conditioned by their own sexuality. Mahon concludes with a look at the eroticism of Bill Viola's quasi-religious video The Passions, which to her justifies the initial assertion that reason and passion are one, and that erotic art will continue in its role of undermining our assumptions about culture and society.

The ultimate lesson to be drawn from her book, however, is the simple realisation that we will need erotic art for as long as we need to attempt to understand our own sexuality. Or to put it in layman's terms, for no other reason than it's sexy.

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