MERCIMONIO. — In general, it indicates any illicit trade, regardless of the source of its illicit nature. In a narrower sense, it refers to illicit trade because it involves prohibited goods; or because, due to their intrinsic dignity, such goods are incompatible with the status of a tradable commodity, or because the law so disposes for the protection of public or private welfare.
Any contract concerning a prohibited object is null; therefore, the mutual obligations that the contracting parties, though freely, believed they had assumed do not exist. However, after fulfillment of the contract, the parties may, in conscience, retain the advantages they have received without any obligation to return them. This is not by virtue of the preceding contract, which remains, due to its invalidity, ineffective; rather, it is by the very fact of the fulfillment of the contractual promises. Such fulfillment, though neither licit nor obligatory in itself, nevertheless fulfills the condition upon which another unnamed contract, “do ut des,” would always be implicit in any illicit transaction. For this atypical contract, the other party owes the consideration—provided it does not consist of an intrinsically illicit act—to the party who actually performed the agreed-upon sinful act; for example, the agreed-upon price.
It is clear that the concrete forms of mercimonio can be very numerous. Within the scope of moral theology, however, the term primarily evokes two: prostitution and the trade in Masses. The evident dignity of the matter exchanged in both forms of mercimonio stigmatizes them both. Even if positive law, deeming it necessary to avert a greater evil to social life, tolerates prostitution, it does not thereby render it morally licit for either those who practice it or those who avail themselves of II. Nevertheless, according to the general doctrine outlined above, the price of infamy may be retained in good conscience. This is also confirmed by a decree of the Sacred Penitentiary of April 23, 1822. Other aspects of this distressing phenomenon are not relevant here (see PROSTITUZIONE and the bibliography cited there).
For the trade in Masses, see ELEMOSINE.