A Mind of Its Own: A Cultural History of the Penis
by David M Friedman
368pp, Robert Hale, £20

In his discussion of ideas of manhood in ancient Rome, David M Friedman tells us that a Roman boy was often given a bulla, "a locket containing a replica of an erect penis", to wear around his neck. "Known as a fascinum, this penis replica signified the boy's status and power... Today, 1,500 years after the fall of imperial Rome, anything as powerful or intriguing as an erection is said to be 'fascinating'."

A Mind of Its Own is fascinating - in other words, as powerful and intriguing as an erection. It argues, very persuasively, that our understanding and appreciation of the penis has passed through a number of distinct and significant phases. These phases one might summarise as the age of the supernatural or religious penis, the age of the scientific penis, the imperial and racist penis, the psychological penis, the ideological penis, and our current phase or age, what we might call the age of the leisure or entertainment penis.

If this argument weren't attractive and intriguing enough, there is the mere fact of Friedman's dealing at all seriously with the subject of male genitalia. These days we are used to all sorts of absurd subjects being treated with deadly seriousness - what's on television, for example, the Northern Ireland assembly, and the work of Damien Hirst. Genuinely important subjects, such as the penis, how to make money and be happy, and the death of civil society, remain objects of misunderstanding, derision and mystification.

Admittedly, Friedman can't resist the occasional fnaar, fnaar. "Freud," he writes, "put the penis on the lips and minds of nearly every educated person in the western world." But such lapses into the realm of the double entendre are rare. A Mind of Its Own is very sure of its own mind, is quite deadpan and benefits enormously from taking itself so seriously.

It is not, let me make it perfectly clear before you rush out and buy it as a novelty Christmas gift, a book of knob jokes. Readers of Viz magazine, fans of David Baddiel, Frank Skinner, Graham Norton and other purveyors of the literal, proverbial and metaphorical chocolate willy, from boy bands to puritanical female newspaper columnists, will probably find it about as interesting as ironing their own underpants.

Friedman makes it clear from the outset that when he writes about the penis he is talking not merely about "the penile shaft and glans" but also "the testes, sperm, and all the other parts and products of the male genitalia". This allows him to cover subjects such as castration, circumcision, impotence, masturbation, testicle implants and testosterone. The emphasis throughout is on the penis as the embodiment and expression of certain obvious powers and possibilities - its erectile functions and misfunctions.

A Mind of Its Own is also, in its way, a work of philosophy. Friedman's central questions are these: "Who are we? What are we made of? How did we get here?" And "Is the penis the best in man - or the beast?" In answer to this last question, cultures seem to swing, as it were, from one extreme to the other.

Friedman begins with the positive examples of the ancient Egyptians, the Greeks and the Romans. Anyone in possession of a penis, an encyclopaedia, an old-school tie, a degree from one of the better universities or a passing acquaintance with Foucault will probably already be familiar with the stories of the Egyptian god Atum creating the universe through an act of "sacred masturbation", with the myth of Osiris and Isis, and with the Greek practice of pederasty. But most people, apart from the devout or the very odd, will probably not be aware of the full range of Roman and Greek penis parades and ejaculation ceremonies, or that "Buddha was said to have a retractable member resembling that of a horse", or that a "book on Hindu aesthetics declares that Siva riding on a bull must be portrayed with an erection reaching his navel".

The Christian penis is, however, much more of a problem. "Clement of Alexandria compared semen to the froth of the epileptic", while "Tertullian taught that it is not merely semen that leaves the penis during orgasm, but part of a man's soul". Augustine, as ever, has a lot to answer for.

The chapter on the Renaissance interest in and study of the penis is particularly interesting, drawing attention to the work of the Dutch autodidact Antony van Leeuwenhoek, whose discovery of the existence of spermatozoa is - if Friedman is to be believed - one of the greatest of modern scientific breakthroughs. The chapter on faked "phallometric" data and the white man's fear of the macrophallic black is appropriately appalling, as is the chapter on feminism and the growth in the idea of the penis as an inconvenience. As for the chapter on Freud, "The Cigar", well, Freud is Freud, and a cigar is a smoke. You have to read the pages about the penile plethysmograph - a medical instrument that measures increases in physical volume - to believe it.

Friedman offers many true tales of the adventures of what Walt Whitman called the "thumb of love". Perhaps the most shocking concerns an off-the-record briefing given by President Lyndon Johnson, who became frustrated with reporters asking him why America was still fighting in Vietnam. Mr President apparently unzipped his trousers, pulled out his penis and said: "This is why!" Bear that in mind next time you hear Blair and Bush bleating on about their special relationship.

· Ian Sansom's The Truth About Babies is published by Granta