Tomás de Mercado's work signified the appearance of one of the first Economics texts that moved away from religious discussion. This is particularly noticeable in the first three Books. Nevertheless, Book 4 more closely follows the tradition of its time, and the weight of the religious and legal perspective surpasses that of its economic approach. For this reason, we have thought it desirable only to translate such chapters (XIII, XIV and XV) as actually have a net economic content. In them, Tomás de Mercado asks the following questions: What is the person who owes an amount of money and cannot pay it without revealing that they have committed a crime or moral fault to do? What must the person who discovers that they have bought a stolen article do? Must the damages caused by war be restituted? What must be done when it is understood that an unfair and unlawful agreement has been made in the past? And who may claim the monies found, for instance, at sea?