If Pride And Prejudice has an extraordinary hold on the imagination of women - and every survey suggests it does - one reason for our obsession is the nature of its hero, Fitzwilliam Darcy. And if the televised version of the novel, with Colin Firth in the role, captivated half the women in the country, the release this Friday of the film version of the novel, with Keira Knightley as Elizabeth Bennet and Matthew MacFadyen as Darcy, should have much the same effect.

Women's infatuation with Darcy is, by and large, a mystery to men. When record numbers of female viewers watched Colin Firth emerge dripping from a lake - a scene that would have been wildly out of place in the novel - male commentators tried to explain the appeal of the whole thing by comparing their girlfriends' fixation with their own about the World Cup. In fact, I know one otherwise sane woman who bid £500 for the irresistible white shirt he wore for that very scene in a charity auction.

But for all the female obsession with the character, it's remarkable how some women - including Knightley - get it wrong about Darcy. Praising MacFadyen's sheer sex appeal, she says, "you need to see that kind of rugged beauty in Darcy, to know that here is a man who strides across fields ... and very much manages his own estate. With Matthew, you can see that etched across his face, yet he's also got this extraordinary vulnerability. On the page, Darcy reads as being very cold, but Matthew is so vulnerable through his big manliness, that he gives Darcy extra qualities."

I'd dispute that. Darcy has endless vulnerability in the novel - and his coldness is merely the cover of a shy, proud man. It's precisely the apparent aloofness of Darcy - think smouldering volcanoes under icecaps - that is crucial to his appeal.

In the novel, Darcy is a rich, proud gentleman with a large estate in Derbyshire and an income of £30,000 a year. We encounter him at a ball, very much in the shadow of his outgoing friend, Mr Bingley.

"Mr Darcy," we are told, "soon drew the attention of the room by his fine, tall person, handsome features, noble mien, and the report which was in general circulation within five minutes after his entrance, of his having 10,000 a year. The gentlemen pronounced him to be a fine figure of a man, the ladies declared he was much handsomer than Mr Bingley, and he was looked at with great admiration for about half the evening, till his manners gave a disgust which turned the tide of his popularity; for he was discovered to be proud; to be above his company, and above being pleased; and not all his large estate in Derbyshire could then save him from having a most forbidding, disagreeable countenance."

And he duly bears this out by dismissing Bennet, in her hearing, as only "tolerable" in the way of looks. Naturally, he revises his view almost immediately, but the damage is done.

As events develop, Darcy finds himself becoming more and more taken with the mixture of archness and sweetness that is Bennet. When she comes to stay at Bingley's house, he is drawn to her against his will. Her vulgar connections, her more than vulgar mother and younger sisters, conspire to make her an unlikely object of his affections. She teases him, and she is, we observe, more effective in capturing his attention precisely because she does not give a fig about his opinion.

Bennet's treatment of Darcy thus bears out the contention of books like The Rules about the relations between the sexes - treat 'em mean and keep 'em keen - because we are quite conscious that had she pursued him rather than mocked him he would not have found himself attracted to her. For a man in his position it is very necessary that he should be chastened by encountering a woman who is indifferent to his looks and status. As Bennet finally declares, "The fact is, that you were sick of civility, of deference, of odious attention. You were disgusted with the women who were always speaking and thinking for your approbation alone. I roused and interested you because I was so unlike them." Spot on, Miss Elizabeth.

But in order to compound the prejudice - of the title - which Elizabeth nurses towards him, her wilful deception by Mr Wickham must come into play; a military gentleman who tells her that he was unjustly denied preferment in the Church of England because of Darcy's animosity towards him. Elizabeth's disdain for Darcy for his arrogance is compounded by her indignation at his treatment of Wickham. And when she discovers that he was the means of separating his friend Bingley from her delightful sister Jane, her resentment is unbounded. The scene is set, then, for Elizabeth's first, indignant rejection of his offer of marriage, which is made, she observes, by a man who expects to be accepted.

So it is that pride gets its fall, and Darcy is sent packing, after declaring his passion for a woman whom he admits he loves against his better judgement. That scene in the film takes place against the sensually potent backdrop of a temple in the estate of Lady Catherine de Bourgh, in a rainstorm. MacFadyen, like Firth, is never more alluring than when he is soaked to the skin.

But no sooner does Elizabeth send him about his business, than he disarms her with a letter that explains why he detests Wickham, as the man had tried to seduce his young sister. She finds she has been unjustly prejudiced, though she is still far from being in love.

That love comes later, when she finds herself by chance in his Derbyshire home. She naturally falls for the sheer elegance and wealth of the estate - a very human reaction - but she is also taken by the testimony of his housekeeper about his goodness to the poor and to his tenants. She encounters Darcy on home ground, where his real worth is made clear.

The reassessment is complete when he rescues the reputation of her sister, Lydia, who had run off with the abominable Wickham. Now it is Elizabeth's turn to have her pride humbled, and when he asks her to marry him, she receives his offer "with gratitude and pleasure".

The beauty of Darcy is that he reassures women of their transformative powers. He is a reminder that the right woman may bring out the emotional depths in a reserved man and can humble the pride of a rich one - though, as in all her novels, Jane Austen is blunt about the economic realities underlying relations between the sexes. As Darcy ruefully acknowledges, at 28 he thought "meanly of all the world", and his pride is duly humbled by his passion for a good woman. "What do I not owe you?" he exclaims. You know, most women would quite like to hear that. It's one of the manifold reasons why his appeal won't ever really diminish.