Descartes’ thought is characterized by a tension between faith and reason with which the philosopher never fully comes to grips. In spite of his many criticisms of the tradition and his endeavor to proceed from entirely new, entirely rational foundations, Descartes appeals in a number of his writings to the importance of faith in human life and the inner compatibility between faith and reason. Nonetheless, as Jacques Maritain (1944, 44) has observed, Descartes’ “fundamental principles will develop into a sheer enmity of reason against faith,” and Descartes’ legacy will eventually become that of a thinker interested exclusively in the earthly project of rendering human beings “the masters and possessors of nature” (AT VI 62, CSM I 142f).

Like Saint Augustine and Saint Thomas Aquinas before him, Descartes proclaims an inner compatibility between religious faith and the principles of his philosophy. He asserts his “extreme devotion to the Catholic religion,” his “firm faith in the infallibility of the Church,” and his conviction that the relation between the truths of faith and the insights of his philosophy are such that he “cannot have any fear that one truth may be in conflict with another” (AT III 259, CSMK 161). Nonetheless, from his earliest writings he seeks to bifurcate the domains of faith and reason along the lines sketched by Galileo in his 1615 “Letter to the Grand Duchess Christina” (Galileo 1957): faith concerns matters pertaining to our salvation, whereas reason pertains to earthly questions that can be settled by argumentation and empirical investigation. Truths such as the mystery of the Trinity and the immortality of the soul are disclosed not by reason but by revelation (AT III 167, CSMK 166; AT V 53, CSMK 320), whereas metaphysical and scientific questions are to be settled by reason and empirical investigation. One of the principal concerns of Cartesian metaphysics is the nature and existence of God. God plays a key role in Descartes’ metaphysics, guaranteeing the truth of clear and distinct ideas; thus natural theology plays a key role in Descartes’ thought. But Descartes scrupulously avoids revealed theology, arguing that it lies outside the domain of his immediate philosophical concerns.