Thomas Aquinas regularly claims that metaphysics is not merely scientific, but the highest and most certain of all the sciences, and his conception of metaphysics is one of the boldest and most epistemically ambitious in the history of philosophy. This book presents a new account of Aquinas's metaphysics, approached from the perspective of his theory of science and knowledge. It offers a novel interpretation of his understanding of the properties of being, the principles of being, the requirements for demonstrative knowledge, and shows how Aquinas's account of metaphysics was able to meet those requirements in a more coherent and compelling way than any thinker who had come before him. It will be of interest to scholars of medieval philosophy, the Aristotelian tradition, metaphysics, epistemology, and philosophical methodology.