CONSENSUS. — In its broadest sense, consensus is the adhesion of the will to an idea presented by the intellect: it is a necessary element for a human act to be called free and voluntary and thus to enter into the moral order (v. HUMAN ACT). A human action is voluntary insofar as it proceeds from a deliberating will, that is, illuminated by the intellect which perceives the relation of the act to its end. All actions performed without the deliberation and consent of the will (in madness, in grave fear that disturbs reason, in violence that compels a reluctant will, in primitive impulses, in sleep, etc.) are neither voluntary nor free and are therefore not susceptible to moral evaluation. Likewise, actions in which deliberation and consent are not full and perfect are not perfectly free and voluntary and are less culpable (v. SIN, mortal and venial).
In law, consensus is spoken of in two distinct meanings: in the sense of the meeting of two wills that agree to regulate one or more legal relationships, and in the sense of approval, by a person having authority (public or private), of another’s decision or act.
In the first meaning, the notion of consensus is relevant in the matter of contracts (v. CONTRACT). Indeed, in contrast to the Roman principle according to which the principal mode of transfer of ownership was traditio rei, the delivery of the thing, the radical and great innovation of modern codes, which derive from the Napoleonic code (Albertine, Italian), consists in having fixed the moment and effect of the transfer of property or a right in the legitimate manifestation of consensus. This innovation is a conquest both logically and juridically: in fact, a contract is essentially and juridically a bilateral transaction, and it exists already in the meeting of two wills which, being internal spiritual faculties, cannot be seen, evaluated, or have social effects unless they are manifested: in the manifested consensus. One thus understands the classical Roman definition of contract: *Conventio duorum pluriumve in idem placitum consensus*.
The external manifestation of the will or internal consensus can occur in different ways and by different means: expressly or tacitly (even silence can manifest a will when there are circumstances such that to remain silent is equivalent to an explicit response; such equivalence exists, however, only when the one who is silent would in any case have had to express consent or dissent explicitly).
It may happen that a will different from that actually existing in the subject’s consciousness is manifested: this occurs either through mental reservation (v.), or through error (v.), or through simulation (v.).
It has been said that in modern law, consensus by itself transfers rights; but if the contract is made between absent parties, when is consensus considered perfected? The principles of the Italian Civil Code can be summarized as follows: the agreement is considered reached and thus the contract concluded at the moment when the party who made the proposal has knowledge of the acceptance of the other party (art. 1326). Such knowledge is presumed at the moment when the acceptance reaches the address of the proposer, unless the latter proves that, without his fault, he was unable to take cognizance of what had reached him (art. 1335). Until the contract is concluded, the proposal and acceptance can in principle be revoked, except for compensation to the party who, in good faith, has begun performance (art. 1328); however, when the contracts involve unilateral obligations on the part of the proposer (suretyship, offer of surety), the proposal is irrevocable once it reaches the knowledge of the recipient (art. 1333).
The defects of consensus, enumerated by jurisprudence, are causes that influence the formation of the act of will so that, without them, the consensus would not have existed or would have been different. In this case, there is thus an act of will that is actually formed and manifested, and another that is neither formed nor manifested, but which would have been formed and manifested had the cause not intervened. Such causes are: error, fraud (v.), violence (v.).
As regards consensus in custom and in marriage, see the respective entries.