This path-breaking approach to Thomas Aquinas interprets the Five Ways in the context of his theory of science. Aquinas is the leading medieval philosopher and his work is of continuing contemporary relevance. Addressing all the critical themes of authority and reason, Christopher Martin examines the role of science and definitions in medieval thought, and how to deal with the big question: is there a God? Rigorous and challenging, Martin's clear exposition compares and contrasts Aquinas' arguments with those of other philosophers, Anselm, Descartes and Kant.