An Analysis of St. Thomas Aquinas’s Position on the Relationship between Justice and Legality

St. Thomas Aquinas, the Angelic Doctor, was renowned for his genius. His masterful exposition of Christian theology, drawing upon Aristotle’s philosophy to form a solid intellectual foundation for Christian ideas, has formed the core of the Catholic Church’s teaching for almost a thousand years. Beyond Christianity, Aquinas has also exerted a major influence on legal philosophy. As the foremost expositor of natural law theory, natural law theorists to the present age owe a great debt to Aquinas’s discussion of law in his Treatise On Law. However, one aspect of Aquinas’s legacy has generated an inordinate amount of contention – his view of the relationship between justice and legality. Is an unjust law a law at all? Modern natural law theorists taking contradictory positions all claim to be faithful to Aquinas’s ideas on the matter. Some, taking a view that has been described as “strong natural law theory”, argue that Aquinas believed that the injustice of a law renders it legally invalid. Others, taking the position of “weak natural law theory”, argue that Aquinas perceived an unjust law as legally defective but still a law in a secondary sense. Even legal positivists, the traditional opponents of natural law theory, have proposed interpretations of Aquinas’s thoughts on the matter. They cannot all be correct. This paper is directed at investigating what really Aquinas’s position was on the relationship between justice and legality. It will do so by taking a detailed look at Aquinas’s Treatise On Law, the broader context of the Summa Theologiae (“the Summa”) within which the Treatise is situated, and Aquinas’s methodological and definitional approaches. The outline of the paper is as follows. First, it will describe the different positions that modern natural law theorists have taken on the relationship between justice and legality, with a view to identifying the key conceptual differences between strong and weak natural law theory. Second, it will briefly describe how Aquinas’s thought on this issue has been interpreted by modern legal theorists, to illustrate the extent of disagreement between interpreters of Aquinas. Third, it will provide an overview of Aquinas’s arc of argument in his Treatise On Law, to provide context for the subsequent deep-dive into Aquinas’s thought. Fourth, the paper will formulate a set of questions to be resolved, corresponding to the key conceptual differences between strong and weak natural law theory, and will then evaluate Aquinas’s text, context, and methodology to determine how Aquinas’s thought bears on these questions.