This book argues that science and metaphysics are closely and inseparably interwoven in the work of Descartes, so that the metaphysics cannot be understood without the science and vice versa. In order to make his case, Thomas Vinci offers a careful philosophical reconstruction of central parts of Descartes' metaphysics and of his theory of perception, each considered in relation to his epistemology. Vinci aims to show that Descartes gave an important positive role to sense-perception in his epistemology, and also that he used his reflections on sense-perception to frame his criticism of previous theories of the sensory qualities of objects.