Mad Madge: Margaret Cavendish, Duchess of Newcastle, royalist, writer and romantic, by Katie Whitaker

Bitter medicine or a refreshing tonic? Min Wild on the writer who arrived just a little too early to find her form

There are a number of ways to take your Margaret Cavendish. You can have her as spice, a dotty and refreshing English eccentric, as did Samuel Pepys and the little boys and girls who followed her carriage through Restoration London. To others she is bitter medicine, indicating - by unique counter-example - women's subservience to men in the 17th century. A few of her contemporaries saw her writings as indigestible goo, something like sago: "verbose and tiresome". Virginia Woolf blithely called her "crack-brained and bird-witted". It is important, then, to approach her with a level head, and this Katie Whitaker does in a substantial, well-researched and comprehensive biography.

It's a question of a confusion of hierarchies: this is how some 17th- and 18th-century upper-class women writers were able to edge open the door to literary recognition. As a marchioness, by virtue of her marriage to William Cavendish, and later a duchess, Cavendish was able to give that door a hefty, insolent shove. In the context of a society which expected women to be modest, silent, and never to speak on the masculine topics of politics or philosophy, Cavendish was an outrageous anomaly. Women were not to aspire to literary fame, and the association of selling your books with selling your body meant that to do so was indecent. Whitaker quotes a man writing to his sister: "your printing of a book, beyond the custom of your sex, doth rankly smell. What will you make yourself to be?" You had to be wary, though, of making such insinuations about a duchess after the Restoration.

To make yourself the sole topic of any writing, for men or women, was only beginning to be acceptable: Montaigne did it, but - to be cavalier about it - he got away with murder because everyone liked him. Cavendish, armed with astounding aristocratic hauteur, wrote her own autobiography at the age of 33. In it she turned on "censuring readers" who will ask "Why has this Lady writ her life? Since none cares to know whose daughter she was, or whose wife she is?" The answer was breathtakingly simple: "'Tis of no purpose to the readers, but it is to the authoress, because I write it for my own sake, not for theirs." "My ambition," she said elsewhere, "is restless, and not ordinary: because it would have an extraordinary fame." She was frank about her reasons for writing: "since all heroic actions" and "public em- ployments... are denied our sex in this age".

Whitaker points out that only 42 new books by women were printed between 1600-1640. Between 1653 and 1673 Cavendish published 23, including plays, short fiction in prose and verse, letters, essays, a biography, six philosophical treatises and a Utopia. Margaret Cavendish and the full-blown novel were made for each other, but the timing was at least 100 years out. It's mildly hilarious, in 2003, to see her admonishing herself - "The world hath already such a weight / Of useless books" - but characteristically she went ahead anyway. Perhaps she took a posthumous lesson from her brother by marriage, whose widow sold his mathematical manuscripts "by weight to the past-board makers for Wast paper". The sage and delightful John Aubrey added that this is "a good Caution for those that have good MSS to see them printed in their life-times".

It may be impossible not to feel ambivalent about the Duchess of Newcastle. Whitaker conjectures that she was dyslexic, but it's pretty hard to tell when Cavendish declared that "it is against nature for a women to spell right." At times she appears to have been downright lazy - though exiled in Paris, she could not be bothered to learn French, and, in one of her Sociable Letters, it seems characteristic that she would not venture to ice-skate unless she was "one of the skilfullest and most practised" - so she didn't. However - and here she turns likeable again - she went home and wrote, with practised skill, an effective and ingenious account of her thoughts as skaters, moving on "smooth glassy ice", some staggering drunkenly, and others sliding "with a good grace and agility". Yet she was so often self-indulgent that reading her work in bulk can make you feel like a slug on warm tarmac.

Where she was most remarkable, as John Rogers has shown, was in her writings on science - more properly for the time, "natural philosophy". Acquainted with Hobbes and Descartes, she formed a theory of intelligence in all material things which is less risible than it sounds, and she was the first woman to be invited to visit the newly formed Royal Society.

Cavendish, like many in the 17th century, enjoyed a far-fetched analogy. One of her own comparisons was between human callings and books: certain volumes, she explained, are like ambassadors, or hangmen or mountebanks. By this analogy, Whitaker's biography is like a sensible and thorough tour guide, patiently walking readers through the colourful vicissitudes of the monarchist aristocracy during the English Civil War. It's readably and cheerfully written, with only an occasional whiff of the banal; and Whitaker is very good on Cavendish's scientific interests. On literary matters, she has little to add to existing work, sometimes assessing rhetorical commonplaces as singular to Cavendish. She has, though, valiantly unearthed new historical material, including more evidence of the richly deserved unpopularity of John Lucas, Cavendish's autocratic brother - this does not, of course, feature in Cavendish's autobiography. Whitaker shows, too, that Cavendish herself was not above some spectacular upper-class boundery.

Charles Lamb loved Cavendish for her "fantastical and original brain", and chose her for his ideal imaginary dinner companion. Whitaker thinks that it may have been his affectionate nickname for her - "Madge Newcastle" - that gave rise during the 19th century to her nickname of "Mad Madge": there may well have been no source for it in her lifetime. So why does this book, clearly designed to place Margaret Cavendish's reputation on a sound footing as an intelligent, sophisticated, bold, original thinker, take the derogatory nickname as its main title?