In this course, we will look at the progression of Descartes’ thought as it was prompted by his encounters with others. To begin, we will read his masterpiece, Meditations on First Philosophy (1641), and then most of the objections he received from several luminaries of seventeenth-century European philosophy and science—Johannes Caterus, Marin Mersenne, Thomas Hobbes, Antoine Arnauld, and Pierre Gassendi—along with his replies. We will then look at Descartes’ 1643-1649 correspondence with Princess Elizabeth of Bohemia. It is difficult to overstate how important the encounter with Elizabeth was for Descartes’ thinking, which changed significantly between the Meditations and The Passions of the Soul (1649), his last major work. Finally, we turn to The Passions in order to examine Descartes’ attempt to take account of the emotions.