Beginning in the Middle Ages, theologians, following the framework established by Aquinas, examined the legitimacy of taxation through four core questions: its licit cause, its necessity and use, the subject and object of taxation, and its proportionality. Taxation was understood as an extraordinary measure which the monarch had to negotiate with the representative assemblies of his realm. This view underwent significant changes during the Early Modern period. Increasing fiscal burdens sparked intense debate amongst Iberian Scholastics. Some theologians and canonists argued that tax fraud was not necessarily sinful, particularly when taxes were levied on the poor or violated customary law. Others emphasized the duty to obey civil law in conscience and affirmed the sovereign’s authority to increase taxation, thereby diminishing the contractual power of the Cortes. The chapter will contextualise these controversies up to the seventeenth century before focusing on the work of the Jesuit Juan de Lugo (1583–1660) in his theological treatise De iustitia et iure (1642). This study also traces the evolving semantics of Late Scholastic discourse in response to both questions raised in auricular confession as well as the political exigencies of the Spanish Empire during wartime.