One of the essential elements in any legal act in Roman law was the volition of the parties involved. It was impossible for a person who did not will to exchange something of his property for another thing of value to him, to exchange it, and so forth. Of course, this volition needed to be expressly manifested, and clearly enough so that it did not create confusion as to what the intent of each of the parties was. This manifestation could occur in several ways; sometimes it was something as simple as behaving in a certain way (like when a person abandoned something, making that object a res derelictae and causing the former owner to lose all property rights over it), and sometimes it was destined to make it known to other people, like when there was verbal stipulation involved.
Of course, and even if it was the norm for the manifestation of volition to match the "internal" intent of the person, sometimes this manifestation was missing, or did not match that internal intent. Here is when we can speak of a vice of consent or will, or of its manifestation.
The vices of manifestation of will could be conscious or unconscious. For instance, manifestations of will made verbally in jest (iocandi gratia) did not have legal efects because the person who said it and those who heard it all knew that it was not meant seriously, as it happened with jokes or theatrical representation. There is also mental reservation, which happened when the person who was manifesting their will did not do it in agreement with their true intention, thus hiding it. This, clearly, did not cause legal effects as long as the other parties involved were ignorant of the vice.
Vices of consent itself, though, were different and did imply legal effects. The three vices of consent in Roman law were violence, error, and dolus.
There is a vice in consent when consent was manifested coercively under threats or physical violence. There was vis absoluta when a third party used physical violence to coerce a person to manifest their will in a certain way, like forcing their hand to sign a contract they did not will to sign. There was vis compulsiva when a third party used threats to cause a person to manifest their will in a way that differed from their true intent. However, for praetorian law, these threats had to abide to certain requirements to be validly considered as threats; it had to be an explicit threat (and not merely implied), it had to be of a nature that it would cause fear for a man of normal strength of character (and not an imbecile or a furiosi), the threat had to be illicit, and it had to be greater than the one that could be derived of celebrating that legal act. The vice of violence caused the nullification of the legal act.
Error as a vice of consent was meant to signify ignorance. This implied not knowing something, or the inexact knowledge of that thing. This could have meant an error about law itself (error in law) which did not excuse the error, since it is impossible to allegate ignorance of the legal norm to nullify a legal act, or an error about a factical matter (error of fact). This last type of error did imply legal effects, as long as it wasn't an error about what anyone could comprehend; about an obvious fact. An error implied the nullification of the legal act, as long as the error was essential, and inexcusable.
Error in negotio: This error is about the true nature of the legal act, and it is always essential, as would have been when a person received a slave under the impression that he was merely acquiring the usufruct, but the owner gave it to him with the intention of selling it.
Error in corpore: This error was about the indentity of the object of the legal act, and it was always essential. This could've happened when a person bought a house thinking it was the one down the street, but the seller actually sold him a house a few blocks away.
Error in substantia: This error was about the substance, or characteristics, of the object of the legal act, as it would have been when a person bought a slave thinking she was a virgin, but really wasn't. This error is not always essential, since it refers to the economic-social function of the object of the act.
Error in quantitate: This error is about the amount of money involved in the legal act, always nullifying it.
Error in qualitate: This error is about the characteristics of the object which do not involve its substance, or its economic-social function, but merely unimportant details of it (like color), thus not causing legal effects of any kind.
Dolus was the evil intent of making a person do something by deceit. For Romans this was the dolus malus, which viced the consent of a person, making them fall in error about one of the aspects of the legal act, which, if they knew, would have caused them to take the determination of not going ahead with that act, or going ahead with it under different conditions. In classic Roman law, dolus was not a cause of nullification of the legal act, only permitting a person to insert a clause in a contract called clausula doli which binded both parties to act without dolus in executing the legal act. However, praetorian law introduced certain remedies for dolus; the actio doli, exceptio doli, and restitutio in integrum propter dolum, all which were legal actions that could be invoked by the injured party looking for retribution for the damages, or nullification of the legal act.
These vices of consent are very much still alive in most legal systems of Roman tradition, as is the Paraguayan civil code.
Petit, Eugéne. Traité élémentaire de droit Romain. Paris, A. Rousseau – 1925.
Caramés Ferro, José M. Curso de Derecho romano. Editorial Perrot. Buenos Aires, Argentina – 1977.
Ghirardi, Juan Carlos. Manual de Derecho Romano. Ediciones Eudecor. Buenos Aires, Argentina – 1982.