Ruhun Tutkuları

Descartes's last book, written partly in correspondence with Princess Elisabeth of Bohemia, who had pressed him hard on the obvious hole in his system: if mind and body are different substances, how does either move the other.

His answer involves the pineal gland and is generally considered his weakest, but the work of cataloguing the passions mechanically is where it earns its place — it treats emotion as something with causes rather than as moral failure.