The Correspondence between Princess Elisabeth of Bohemia and René Descartes

An exchange of letters between Descartes and Princess Elisabeth of Bohemia, a learned exile living in The Hague, lasting from 1643 until shortly before his death. Elisabeth's first letter poses the question that still bears her name: how can a soul that is only a thinking thing move the bodily spirits, when all motion seems to require contact and extension? Descartes replies that there are three primitive notions, of mind, of body and of their union, and that the union is best known through the senses and ordinary life rather than through metaphysics. Elisabeth is not satisfied, and remarks that it would be easier to grant the soul matter and extension than to grant an immaterial thing the power to move a body. Later letters turn to Seneca on the happy life, the control of the passions, illness and grief, providence and free will, and Machiavelli's Prince. The discussion fed directly into Descartes's Passions of the Soul.

It contains the sharpest early statement of the mind–body interaction problem, made by a philosopher who never published, and shows philosophy being done as a real exchange between equals.