Between Two Worlds is an authoritative commentary on--and powerful reinterpretation of--the founding work of modern philosophy, Descartes's Meditations. Philosophers have tended to read Descartes's seminal work in an occasional way, examining its treatment of individual topics while ignoring other parts of the text. In contrast, John Carriero provides a sustained, systematic reading of the whole text, giving a detailed account of the positions against which Descartes was reacting, and revealing anew the unity, meaning, and originality of the Meditations. Carriero finds in the Meditations a nearly continuous argument against Thomistic Aristotelian ways of thinking about cognition, and shows more clearly than ever before how Descartes bridged the old world of scholasticism and the new one of mechanistic naturalism. Rather than casting Descartes's project primarily in terms of skepticism, knowledge, and certainty, Carriero focuses on fundamental disagreements between Descartes and the scholastics over the nature of understanding, the relation between the senses and the intellect, the nature of the human being, and how and to what extent God is cognized by human beings. Against this background, Carriero shows, Descartes developed his own conceptions of mind, body, and the relation between them, creating a coherent, philosophically rich project in the Meditations and setting the agenda for a century of rationalist metaphysics.