“I am writing my life to laugh at myself, and I am succeeding,” Casanova explained to a friend in 1791. The resulting 3,800-page memoir was much more than a source of personal merriment: it established the author's reputation as the most famous womaniser of all time and bequeathed to posterity a wealth of information about the social and cultural mores of 18th-century Europe.

Casanova's The History of My Life begins with his birth in Venice in 1725, the child of an actress outside the realms of social acceptance, and ends in 1774, when he was 49: old by 18th-century standards. He died in 1798, before he finished his memoirs, but late enough to have seen the French Revolution destroy the world, and many of the aristocratic friends he had known in his itinerant youth. Posthumously published, pirated, abridged and poorly translated from the French in which Casanova wrote, his memoirs nevertheless took on a life of their own: the idle braggart and erotic fantasist of legend was born.

Any biographer of Casanova faces at least two enormous problems: how to step out from the shadow of the gargantuan memoirs, and how to assess their truthfulness, when Casanova's own word about his amorous escapades is all the evidence that remains.

Ian Kelly resolves both problems magnificently; an actor himself, he approaches Casanova as the inhabitant of many roles: actor, spy, lover, priest and, above all, writer. Where others have berated Casanova for exaggeration, inaccuracy or manifest error, Kelly extends him imaginative sympathy: writers often lie, and that's before we get on to lovers, but where the history of emotions is in question, the literal truth may be beside the point.

Carlo Goldoni, the Venetian playwright who modelled the dissolute character of Don Giovanni on his friend Casanova, claimed: “Every man is three people; the man as he sees himself, the man as others see him, and the man as he really is.” Kelly is too canny a biographer to attempt a portrait of Casanova as he really was. But he does provide a surprisingly intimate sense of what Casanova was like.

There are broadly two types of womaniser: one motivated by hatred, the other by love. Casanova was the second kind. The difference lies not so much in what happens between the sheets, but in what happens afterwards: Casanova proved himself time and time again capable of lasting friendships with the women he had taken to bed. He was not the sort who cuts and runs, pursued by a dust-cloud of confusion and unanswered questions.

Casanova knew that women's lives were, in general, harder than his own. He wanted to help, took an interest in contraception, donated dowries, procured abortions, and sequestered the haplessly pregnant away from public disapproval. “It was a sophisticated position,” Kelly argues. “He wanted the joys of carnality but also to know that he had left his lovers with a positive memory of their encounter and an empowered position, as women. Even at 17, he rejected notching his bedpost, for his own as well the sake of his ‘conquests'.”

Casanova never married. He did at one point settle down with a common-law wife in Venice, but he left her and ended his days as a writer and librarian in Dux. Earlier, marriage would have impeded his travelling, which was first a form of adventuring, later an enforced exile when he fell foul of the Venetian authorities and had to escape from a five-year prison sentence. He covered an estimated 64,060 kilometres over his lifetime, visiting Paris, London, St Petersburg and Constantinople among other cities, impressing the local literati and grandees, winning and losing fortunes, and trailing a reputation as a spiritual healer steeped in the mysteries of kabbala.

Contemporary celebrities have renewed general interest in the kabbala. Kelly describes its 18th-century presence as a “fusion of Gnosticism, Egyptian mathematics, neo-Platonism, Judaic mysticism and personal revelation.” He does not try to explain (or explain away) Casanova's attraction to the kabbala, but speculates that it must have been a combination of pragmatism and faith. This is an ingratiatingly liberal approach, consistent with the view that the biographer's role is to understand, not pass judgment.

Sometimes Kelly's imaginative sympathy with Casanova is audibly challenged: on the subjects of incest and paedophilia, for example. Casanova never played a responsible role in the lives of the many children he fathered, and on one occasion went so far as to seduce a young girl he had good reason to believe was his natural daughter. This, taken together with a growing interest in teenage girls as his own old age encroached, seems sordid.

In his defence, all that can be said is that Casanova at least knew, and lamented, the ways in which life had coarsened him. Kelly tells how an early love, one Casanova had thought to marry, scratched on to the window pane of their last hotel room the words: “You will also forget Henriette.” Years later Casanova found himself back in Geneva in the same hotel room: the prophecy in the glass made him shake with fear. It was more than the memory of lost love, for in the interim he had also mislaid “the delicacy which was then mine and exalted feelings which justify the errors of the senses”. Casanova, like Venice itself, was sinking into waves of oblivion.

Casanova by Ian Kelly
Hodder, £20 Buy the book here

Casanova will be Book of the Week on Radio 4 starting Monday, June 23, 2008

Times Recommends

• Superfreakonomics by Steven D Levitt and Stephen J Dubner
• The Times picks the 50 best paperbacks of 2009
• Vincent van Gogh’s letters show the soul of the artist