In 1650 the Cambridge philosopher Henry More began teaching philosophy by letter to the half-sister of one of his students. The university admitted no women. The course he set her was built on the newest system in Europe, that of René Descartes. Descartes divided the world into two kinds of thing: minds, which think, and bodies, which only take up space and move when something pushes them. More kept a division of his own between spirit and body. His pupil, born Anne Finch and from 1651 Anne Conway, worked on the relation of spirit and body for the rest of her life and ended by rejecting both teachers. She argued that dead matter does not exist. Everything created is alive and perceives in some degree, and body and spirit are one substance, which differs only in how coarse or how fine it is.
She reached that position at Ragley Hall, her husband's house in Warwickshire, and she reached it in pain. Headaches had troubled her since her youth, and they grew worse and more frequent as she aged. William Harvey, the physician who discovered the circulation of the blood, was among the doctors who treated her without result. In 1656 she travelled to France, with More in the party, to have her skull opened; the operation was not carried out. Ten years later the Irish healer Valentine Greatrakes, famous for curing by the touch of his hands, tried to cure her and failed. Her only child, a son, had died of smallpox in 1660 at the age of two.
In 1670 Francis Mercury van Helmont, a Flemish physician, came to Ragley. He did not cure her, but he stayed until she died. Through him she studied the Kabbalah, the Jewish mystical tradition, and that study helped to break her attachment to Descartes. He also brought her into contact with the Quakers, a religious society whose members were then being jailed across England. George Fox, its founder, visited her, and she used her connections on behalf of those in prison. Shortly before her death in 1679 she became a Quaker herself, against the objections of her family and of More.
The treatise she wrote in her last years starts from God. A God who is good, wise and alive, she reasoned, makes creatures that resemble their maker, so nothing God made can be lifeless. Things, too, can act on one another only if they are alike. If the soul shared nothing with the body, she asked, why should it suffer when the body is hurt? From the likeness of all creatures she drew a further conclusion: over successive lives one creature can become another, and a horse can in time become a human being. Pain belongs to this process as a remedy. It wears away what has grown coarse in a creature and sets it back on the way to recovery. For that reason, and because endless punishment for a limited wrong would be unjust, she denied that hell lasts for ever. Thomas Hobbes and Baruch Spinoza, she argued, had made the opposite error: they had left no difference between God and the things God made.
She published none of it. Van Helmont carried her notes to the Continent, and in 1690 they were printed at Amsterdam in Latin, without her name. Her English original is lost; The Principles of the Most Ancient and Modern Philosophy, printed in London in 1692, was translated back from that Latin. Gottfried Wilhelm Leibniz, the German philosopher and a friend of van Helmont, owned a copy and wrote in 1697 that his own views came close to hers. Scholars disagree about how much his philosophy owes to hers. Because the book carried no author's name, it was little read for almost three centuries. Cambridge University Press published a new English translation from the Latin in 1996.