This volume contains new translations of the essential philosophical writings of Thomas Aquinas from the Summa Theologiae and The Principles of Nature. The included texts represent the breadth of Aquinas's thought, addressing: the fundamental principles of nature; causality; the existence of God; how God can be known; how language can be used to describe God; human nature (including the nature of the soul, free will, and epistemology); happiness; ethics; and natural law. The goal of these translations is twofold: to allow Aquinas to speak for himself, but also to make his thought accessible to the contemporary reader without the burden of unnecessary adherence to convention. A thorough yet accessible introduction is included, as are a series of useful appendices connecting Aquinas's arguments to those of Anselm, Scotus, and others.