Descartes is always and ever concerned with knowledge. Around 1628 he begins his systematic philosophical work by starting to write, though never publishing, the `Rules for the Direction of the Mind' (`Regulae' `ad Directionem Ingenii'). In this work he lays out rules designed to direct the mind in its quest for knowledge. Towards the end of his life, in 1649 he publishes `Passions de L'ame (Passions of the Soul)' in which he considers the ways in which the passions affect our knowledge and how to control them. However, the Galileo affair in 1633 and later his need to reply to Objections to his `Mediations' in 1640-41 provoked crises in Descartes' intellectual development the import of which has not been sufficiently recognized. These events are the major reasons why Descartes' philosophical position concerning how we know and what we may know is radically different at the end of his life from what it was when he began. Indeed, his changing position on the what, how and why of knowledge has major implications for his views concerning God, causality, metaphysics, the nature of science, and even what mind-body dualism can mean. A further meta-implication of this claim is that serious errors await any scholar who cites early Cartesian texts in support of late Cartesian positions, or who uses later texts to support a reading of early ones, to provide a “unified” view of Descartes' philosophy. (gezamenlijk georganiseerd met het Centrum voor Wetenschapsgeschiedenis)