A number of seventeenth-century philosophers grappled with the question of what is the soul, and its related questions of how to know about the soul, and how to gain knowledge about the soul. This chapter begins with an account of Saint Thomas Aquinas's views about the knowledge of the soul. Rene Descartes's theory of self-knowledge is then traced in some detail, for his doctrine provided a new framework within which much of the discussion of the topic was carried on during the rest of the century. Next, an account is given of the views of several thinkers, Nicolas Malebranche, the Cambridge Platonists, and Gottfried Wilhelm Leibniz, who differed from him about the precise nature of the soul and the scope of the knowledge of it. The chapter concludes by looking at Baruch Spinoza, Thomas Hobbes, Pierre Gassendi, and John Locke, the thinkers who disagreed with Descartes about many matters, including the knowledge of the soul.