In the treatise Passions of the Soul (French: Les passions de l'âme), the last of Descartes' published work, completed in 1649 and dedicated to Princess Elisabeth of Bohemia, the author contributes to a long tradition of theorizing "the passions". The passions were experiences now commonly called emotions in the modern period, and had been a subject of debate among natural philosophers since the time of Plato. Notable precursors to Descartes who articulated their own theories of the passions include St. Augustine, St. Thomas Aquinas and Thomas Hobbes.