Broad terminological agreement and a shared appreciation of the role of reason in ethics do not mask the very significant disagreements that led to three distinctive philosophical systems: Cartesian dualism, Spinozan monism and Leibnizian pluralism. The book explores the nature of, and offers reasons for, these differences. Phemister contends that Spinoza and Leibniz developed their systems in part through engagements with and amendment of Cartesian philosophy, and critically analyses the arguments and contributions of all three philosophers. The clarity of the authors discussion of their key ideas including their views on knowledge, universal languages, the nature of substance and substances, bodies, the relation of mind and body, freedom, and the role of distinct perception and reason in morals will make this book the ideal introduction to rationalist philosophy.