  Perhaps the most profound effect that Descartes had on early modern epistemology and metaphysics arose from his idea to examine the knower as a means to determine the scope and possibilities of human knowledge. Among his immediate followers, Malebranche most fully developed this aspect of Descartes' philosophy. Subsequent philosophers who were not followers of Descartes also adopted the strategy of investigating the knower. The epistemological works of Locke, George Berkeley, David Hume, Thomas Reid, and Immanuel Kant pursued this investigation. These authors came to different conclusions than had Descartes concerning the ability of the human mind to know things as they are in themselves. Hume and Kant especially—and each in his  own way—rejected the very notion of a metaphysics that reveals reality as it is in itself. They did not  merely deny Descartes' particular metaphysical  theories; they rejected his sort of metaphysical project altogether. But they did so through the type of investigation that Descartes himself had made prominent: the investigation of the cognitive capacities of the knower.