In response to court cases in Italy, Colombia, and elsewhere, in which people and communities have appealed to the legal doctrine of necessity, this essay examines the moral-theological tradition reflection on the law of necessity (ius necessitatis) and its preservation in Catholic social teaching. While it is commonplace in our own day to encounter arguments that Christianity underwrites private property rights, this tradition of reflection understands such defenses as based upon a fundamental misunderstanding of what property is and what it is for, which is to meet the needs of all people, because God gives creation as a common gift. For this reason, claims of need can take precedence over claims of private property; the former does not violate the law but illumines its central purpose. Thus, the aim of this essay is to rearticulate the basic rationale and theological moorings of the law of necessity, as well to show how Catholic social teaching preserves it, especially in relation to what Gaudium et spes calls the common or universal destination of created goods. However, while social teaching continues to preserve the ius necessitatis, this law, and right it attempts to secure, remains a shadowy one because of its complex and oftentimes fraught relationship to positive law.