This book offers a revisionary account of key epistemological concepts and doctrines of St. Thomas Aquinas, particularly his concept of scientia (science). It proposes a new interpretation of the purpose and composition of Aquinas' most mature and influential work, the Summa theologiae, which has traditionally been regarded as a work for neophytes in theology. John Jenkins' comprehensive and original study will be of interest to readers in philosophy, theology and medieval studies.