 René Descartes’ rationalist system of philosophy is one of the pillars on which Enlightenment thought rests. Descartes (1596–1650) undertakes to establish the sciences upon a secure metaphysical foundation. The famous method of doubt Descartes employs for this purpose exemplifies (in part through exaggerating) an attitude characteristic of the Enlightenment. According to Descartes, the investigator in foundational philosophical research ought to doubt all propositions that can be doubted. The investigator determines whether a proposition is dubitable by attempting to construct a possible scenario under which it is false. In the domain of fundamental scientific (philosophical) research, no other authority but one’s own conviction is to be trusted, and not one’s own conviction either, until it is subjected to rigorous skeptical questioning. With his method, Descartes casts doubt upon the senses as authoritative source of knowledge. He finds that God and the immaterial soul are both better known, on the basis of innate ideas, than objects of the senses. Through his famous doctrine of the dualism of mind and body, that mind and body are two distinct substances, each with its own essence, the material world (allegedly) known through the senses becomes denominated as an “external” world, insofar as it is external to the ideas with which one immediately communes in one’s consciousness. Descartes’ investigation thus establishes one of the central epistemological problems, not only of the Enlightenment, but also of modernity: the problem of objectivity in our empirical knowledge. If our evidence for the truth of propositions about extra-mental material reality is always restricted to mental content, content before the mind, how can we ever be certain that the extra-mental reality is not other than we represent it as being? Descartes’ solution depends on our having secured prior and certain knowledge of God. In fact, Descartes argues that all human knowledge (not only knowledge of the material world through the senses) depends on metaphysical knowledge of God.