This book reassesses the ethics of reason in the Age of the Reason, making use of the neglected category of conscience. Arguing that conscience was a central feature of British Enlightenment ethical rationalism, the book explores the links between Enlightenment philosophy and modern secularisation, while responding to longstanding criticisms of rational intuitionism and the analogy between mathematics and morals, derived from David Hume and Immanuel Kant. Questioning in what sense British Enlightenment ethical rationalism can be associated with a secularising ¿Enlightenment project¿, Daniel investigates the extent to which contemporary, and secular liberal, invocations of reason and conscience rely on the early modern Christian metaphysics they have