Contracts Violation: Justification via Argumentation Stefania Costantini, Arianna Tocchio, and Panagiota Tsintza Dip. di Informatica, Universit` a di L’Aquila, Coppito 67100, L’Aquila, Italy stefcost@di.univaq.it, tocchio@di.univaq.it, panagiota.tsintza@di.univaq.it Abstract. An important field of application of intelligent logical agents, where rationality plays a main role, is that of automated negotiation. Our work is re- lated to the use of argumentation in the field of negotiation. In particular, we are interested in contract violations, and in the construction of justifications to motivate the violation itself and recover if possible the contract on modified con- ditions. We propose a temporal modal logic language able to support and depict the arguments/justification used in dialectical disputes and we consider suitable algorithms and mechanisms to introduce and manage justifications. 1 Introduction Agents living in open societies need to interact in order to decide and coordinate actions that have to be undertaken in order to fulfill a specific and desired goal. In particular, in the case of negotiation (for a survey the reader may refer to [1]), agents have to sign and respect contracts (“deals”) that may fail if one of the parties is for some reason unable to fulfil its commitments. In this paper, we intend to present a computational logic framework where an agent can on the one hand try to justify itself in case of a violation by proposing a justification, and on the other hand can evaluate the arguments received by another party that have not respected a deal. The overall objective is that of recovering, if possible, a failed contract on a modified base, even by paying some kind of penalty or performing suitable recovering actions. This objective is of interest not only to single agents, but to the whole “society” where they “live”. In general terms, a successful negotiation process leads to an agreement, called a contract, between the involved parties. Typically, a “final” or “signed” contract contains commitments accepted by both parts, that have to be respected in order to conclude the agreement. An omission of a commitment leads to the violation of a signed contract. The omission of a commitment by an agent may have several causes: some of them may depend upon the involved agent, such as lack of resources and/or of capabilities; sometimes however, environmental or social obstructions may occur that the agent is not able to cope with. Therefore, an agent that has violated a contract should be allowed to justify itself by informing the opponent about the particular causes that led to this violation. We propose a formalization for justifications, intended as motivations leading to the omission of a commitment, as a particular kind of argumentations. Real-world contracts include a number of sanctions and repair actions to be under- taken in case of a contract violation. That is, to each particular case of violation it is in general possible to associate a set of repair rules aimed at trying to recover the contract. M. Fisher, F. Sadri, and M. Thielscher (Eds.): CLIMA IX, LNAI 5405, pp. 132–153, 2009. c  Springer-Verlag Berlin Heidelberg 2009