89, No. 4. A central thesis in the ethics and epistemology of St. Thomas Aquinas is that both speculative and practical reason proceed from first principles in their search for truth. According to Aquinas, the first principles of both orders are analogous and share common features. However, it is not always easy to understand the extent of this analogy. In this paper we intend to analyze two common properties of the first principles to show similarities and differences that exist in the way these analogous properties are verified in each order of principles. Firstly, we expound which characteristics correspond to the knowledge of both orders of principles. Secondly, we examine the idea that the knowledge of all the first principles comes from sense knowledge. Finally, we address the idea that all other knowledge, of both the speculative order and the practical order, depends on first principles.