Matter, as the new philosophy of the 1640s described it, did nothing by itself. René Descartes held that a body is only extension, moved from outside, and that thinking belongs to a separate substance, the mind. Thomas Hobbes allowed no second substance, but his bodies too moved only when another body pushed them. Both men were guests in Paris of William Cavendish, Marquess of Newcastle, an exiled English general, and of his wife Margaret, who by her own account exchanged little more than courtesies with them. She came to think that both were wrong about matter. Her claim, argued in book after book, was that matter moves itself, and that every part of it senses and knows in some degree.
She had almost no schooling. Born Margaret Lucas, the youngest of eight children of an Essex landowner who died when she was an infant, she was taught at home to read, write, sing and dance, and she never learned Latin, the language of the universities. In 1643 she joined the household of Henrietta Maria, the queen of Charles I, as a maid of honour, and the next year she followed the queen into exile in France. There, in 1645, she married Newcastle, who was thirty years older and had left England after losing the battle of Marston Moor. The Civil War kept costing her family. In August 1648 her brother Charles Lucas, a royalist commander, was shot by a firing squad at Colchester after the town surrendered to Parliament's army. In November 1651 she crossed to London with her brother-in-law to petition for an income from her husband's confiscated estates. The petition failed. She stayed until 1653, and while she was there she began to write for print.
Her first book, Poems and Fancies of 1653, built the world out of atoms. Two years later, in Philosophical and Physical Opinions, she printed a condemnation of that theory (scholars still dispute how complete the break was) and set out the system she kept for the rest of her life. Atoms that neither sense nor think, she argued, could not meet by chance and produce a world as regular as this one. Order requires knowledge, and since she admitted nothing into nature except matter, the knowledge had to belong to matter itself. A hand that throws a ball therefore passes no motion into it: the ball moves by its own motion, and the hand is only the occasion. The human mind is matter as well, so the question that troubled readers of Descartes, how a mind can move a body, does not arise. God alone she placed outside nature, and beyond what reason can know.
Her contemporaries would not dispute with her in print; she arranged the dispute herself. Philosophical Letters of 1664 answers Hobbes, Descartes and two other authors passage by passage, in letters to a correspondent she invented. Observations upon Experimental Philosophy, two years later, argues against the Royal Society, the new London body for experimental science, and against its microscopes: a lens, she argued, shows the outside of a thing, often falsely, and only reason reaches the motions within. She bound a fiction, The Blazing World, into the same volume.
She sent her books to the universities and to scholars. The replies praised her and left the arguments alone. Only Joseph Glanvill, a clergyman who defended the Royal Society, argued back. On 30 May 1667 the Society received her at Arundel House in London; she was the first woman to attend one of its meetings. Samuel Pepys, the diarist, wrote afterwards that he had heard her say nothing worth hearing. She published Grounds of Natural Philosophy, the last version of her system, in 1668, and died on 15 December 1673.
After her death she was remembered as an eccentric; Virginia Woolf, in an essay of 1925, found real fire somewhere in the mass of her writing. Philosophers began to read the arguments in the last decades of the twentieth century, and in 2001 Cambridge University Press published the Observations in an edition by Eileen O'Neill.