The expression ius naturale evokes, in its apparent simplicity, the existence of a relation between Law and Nature. This relationship between the normative and the natural—despite being affirmed in diverse and sometimes incompatible ways—was, for centuries, a relatively pacific question. Criticism of natural law theory, or iusnaturalism, became common in the 19th century, when, under the influence of positivism, the conviction prevailed that natural law lacks the capacity to clarify the concept of law, and still less, to project itself onto the concrete practice of law.

The 19th century critique of iusnaturalism traces its roots to the rationalism of the 18th century. During the 18th century, a view of natural law as rational code arose. This code was to be accurately formulated by the study of human nature. But this study of human nature took the form of an individualistic analysis of man from the perspective of modern rationalism. This perspective (rationalist iusnaturalism) supplanted what might be called a ‘classical’ perspective (classical iusnaturalism), which took man’s finality as the fundamental explanatory criterion, by instead prioritizing material and formal causality as the principal explanatory criteria. In other words, the rationalist perspective is characterized by the conviction that reality is better understood by unraveling the internal composition of its elements and analyzing the relation between them, rather than by investigating the end or purpose of its existence. An investigation into the ‘why’ of reality (the classical perspective) was virtually replaced by an investigation into the ‘how’ of reality (the rationalist perspective). As a result, reflection on natural law lost touch with teleology. Rationalist iusnaturalism became, in turn, legal positivism: the first European legal codes appeared as compendiums of all the natural rules that had to govern human relations. In this sense, the sentence of Cambacérès, when he presented the second project of Civil Code for France, is emblematic: "Our laws will not be but the code of Nature, sanctioned by reason and guaranteed by freedom". This rationalist iusnaturalism is the principal form of iusnaturalism that has stood in opposition to the later positivism of the 19th and 20th centuries, presenting, it as it were, classical iusnaturalism by antonomasia.

This image, represented by rationalist iusnaturalism, is not the object of the proposed Workshop, but rather the classical iusnaturalism that is presently identified as the Aristotelian-Thomistic Natural Law Tradition. This Aristotelian-Thomist jurisprudence developed from a rehabilitation of the study of practical reasoning, in dialog with the contemporary philosophy. On this perspective, natural law is not reduced to its mere historical formulations. If there is a natural law, exists independently of the theories that we could develop about it. Nevertheless, when we talk about natural law we necessarily refer to a specific doctrine, a theory formulated under the philosophical debate surrounding the foundations of the moral order. Although the ancient antecedents of this doctrine are found in Stoicism, its more classic formulation is owed to Aquinas. It is to this version of the natural law that the contemporary efforts at rehabilitating the study of practical reasoning are referred.