At least as far back as Thomas Aquinas, Christian thought has recognized two sources of knowledge of God: revelation and "natural reason". The study of the truths revealed by reason is called natural theology. During the Age of Enlightenment, especially in Britain and France, philosophers began to reject revelation as a source of knowledge and to appeal only to truths that they felt could be established by reason alone. Such philosophers were called "deists" and the philosophical position that they advocated is called "deism".