It is a traditional position of the Methodist Church that any disciplined theological work calls for the careful use of reason by which to understand God's action and will. However, Methodists also look to Christian tradition as a source of doctrine. Wesley himself believed that the living core of the Christian faith was revealed in the Bible as the sole foundational source. The centrality of Scripture was so important for Wesley that he called himself "a man of one book". Methodism has also emphasised a personal experience of faith; this is linked to the Methodist doctrine of assurance. These four elements taken together form the Wesleyan Quadrilateral.