The first principles of natural law: A study of the moral theories of Aristotle and Saint Thomas Aquinas

Maria Theresa Carl, Marquette University


This dissertation is concerned with the problem of how to establish and justify the principles of natural law ethics. Specifically, this work critically examines the claim that the primary principles of natural law ethics are per se known or self-evident propositions. This is the position held by St. Thomas Aquinas, which he derived from Aristotelian logic and philosophy of science. Aristotle held that the starting points or first principles of demonstrative, theoretical science cannot themselves depend on demonstration to be known. Rather, he argued that such principles are immediate and indemonstrable and that they are apprehended through an inductive process which culminates in an intellectual intuition or insight. Aquinas explicitly extends this theory to practical science in his formulation of the primary precepts of the natural law (Summa Theologiae I-II, g. 94, a. 2). There Aquinas sets forth the basic goods which are the natural ends of human volition: i.e., life, society, knowledge. These self-evident goods correspond to the various natural inclinations which define the essence of what it is to be human. The major contemporary ethicians working within the Aristotelian-Thomistic tradition remain unclear about the precise meaning of the notion of self-evidence and disagree about which principles or basic values the criterion of self-evidence in fact justifies. Accordingly, in this work I first explain Aristotle's notion of science and his identification of the indemonstrable first principles of science with essential definitions. Next, I argue that Aristotle properly and unequivocally categorizes the practical sciences, including politics and ethics, as science, and I discuss the nature of practical science and the analogy between speculative and practical science. Third, I examine Aquinas' notion of self-evidence and show how he utilizes this notion to ground the speculative and practical sciences. Finally, I offer an exegesis of Summa Theologiae I-II, q. 94, a. 2 and an account of its significance for Thomas' natural law theory.

Recommended Citation

Carl, Maria Theresa, "The first principles of natural law: A study of the moral theories of Aristotle and Saint Thomas Aquinas" (1989). Dissertations (1962 - 2010) Access via Proquest Digital Dissertations. AAI9014051.