Kepler is mainly known among historians of science due to his astronomical theories and his approaches to problems having to do with philosophy of science and ontology. This thesis attempts to contribute to Kepler studies by providing a comprehensive discussion of a topic hitherto not really considered, namely Kepler’s theory of the soul, a general theory of knowledge or epistemology whose central problem is what makes knowledge possible—rather than what makes knowledge true, as happens in the case of Descartes’s and Bacon’s epistemologies. Kepler’s theory consists of four issues: the theory of the different sorts of soul—i.e. the human soul, the animal soul, the vegetable soul, and the Earth soul—concerning their faculties, the differences and the resemblances that emerge among them, the relation they maintain with their own bodies and the world, and the distinction soul-world. The thesis discusses these issues from a historical perspective, that is, it reconstructs the way they appear in three periods of Kepler’s career: the period prior to the publication of the Mysterium Cosmographicum, the period going from 1596 to 1611, and the harmonic period. Finally, Kepler’s epistemology is briefly contrasted with Descartes’s and Bacon’s in order to suggest why Kepler’s is philosophically interesting and valuable.