Book review: The spirit was thrilling but the flesh was meek Our guide to Sophie's World has impersonated a saint's mistress. Karen Armstrong judges him

Saint Augustine, the founder of Western Christianity, has bequeathed a difficult legacy. By seeing sexuality as a symptom of fallen humanity's chronic sinfulness, he has made it difficult for the churches to keep their balance about sex and gender. He would have been fully in sympathy with those who oppose the ordination of women. "What does it matter," he once wrote to a friend, "whether it is in a wife or a mother? It is still Eve the temptress that we must beware of in any woman."

Augustine's Confessions show us that his bleak view of sex and women derived in part from a long personal struggle. Few events in his tormented life have occasioned as much outrage as his dismissal of the woman who had lived with him for more than 12 years and had borne his son, Adeodatus. He makes it clear that this step, forced on him by his mother so that he could make a good marriage, caused him intense anguish.

Now Jostein Gaarder gives this discarded woman a voice, publishing what purports to be a letter from Floria Aemilia to her former lover, complete with footnotes, giving her side of the story. The obvious model for this epistolary cri de coeur is the heartbreaking correspondence between Abelard and Heloise. Gaarder imagines Floria to be a learned woman, like Heloise, and peppers her letter with allusions to the philosophers.

Yet Floria lacks Heloise's pathos. She takes Augustine to task, arguing with him theologically and is not above teasing him occasionally. When she quotes Augustine's lurid description in the Confessions of the "abyss of passion" and the "maelstrom of vice" into which he plunged in youth, for example, Floria drily remarks that he is bragging. She remembers him as "a rather fumbling and inexperienced fellow".

Even though her story is tragic, the tone of Floria's argument is so relentlessly cerebral that she does not engage our sympathies as much as we would expect. It would be anachronistic to expect her to have modern psychological insight (though Gaarder wittily makes her refer to the story of Oedipus when she describes Augustine's excessive dependence on his mother). Yet the persistently sprightly tenor of Floria's narrative distances her from the reader. She does not let us see the raw grief she must have experienced. And she seems positively obtuse in not realising that Augustine's struggles were taking place in the obscure world of the subconscious (called memoria in the Confessions) and could never be assuaged by her purely rational arguments.

Indeed, the triumph of Gaarder's reconstruction is that the reader comes away from this sad tale with an entirely new sympathy for Augustine. I am not sure whether he intended this. Some of his footnotes indicate that he sees Augustine as a faintly ridiculous figure. Yet the plight of the ageing Bishop of Hippo, wrestling until the end of his life with erotic dreams and with a regret that he can only allow to surface in sleep, is truly tragic.

In her understandably bitter admonition, Floria chides Augustine for a theology which had severed him from each of his five senses and hence from the world. This is ironic, since his religion is rooted in devotion to a deity which took flesh and which should, therefore, see the body as sacred.

Intentionally or not, Gaarder has helped us to appreciate the terrible loneliness of Augustine's vision. In a sermon which Floria does not quote, Augustine speaks of the isolation and alienation imposed on each human being by the flesh in which "we are covered round, through which flesh the heart cannot be seen ... and every heart to every other heart is shut".