Why did Aquinas believe that an unjust law might not create an obligation to obey?

St. Thomas Aquinas argued that the fundamental purpose of law is to serve the common good of a political community. Because he defined law as an "ordinance of reason," he maintained that valid positive law must be derived from objective moral principles, either through direct deduction or through determinatio—the specification of how general moral principles apply to facilitate human coordination.1

Aquinas concluded that if a human-enacted law fails to derive from these valid moral principles, it is considered "unjust". Following the tradition of St. Augustine, Aquinas held that such unjust statutes fail to qualify as true "law" at all. Consequently, because they lack the essential character of law, they do not possess binding authority, and citizens are not under an obligation to obey them. Furthermore, he noted that laws created by authorities who are not motivated by the "care of the community" represent a distinct form of injustice that can also invalidate positive law.1