A Knowledge Representation Language for Defeasible Argumentation 12 Guillermo R. Simari Alejandro J. Garc´ ıa 3 Grupo de Investigaci´on en Inteligencia Artificial (GIIA) Departamento de Ciencias de la Computaci´on Universidad Nacional del Sur Av.Alem 1253 – (8000) Bah´ ıa Blanca, ARGENTINA fax: (54) (91) 553933 Phone: (54) (91) 20776 (ext. 208) e-mail: grs@criba.edu.ar ccgarcia@criba.edu.ar Abstract The goal of this work is to define a system for defeasible argumentation, as an extension of logic programming. Here, we define Defeasible Logic Programs (dlp), and a notion of defeasible inference from a dlp. The characteristics of a dlp allow to represent different flavors of the closed world assumption (CWA) just as program clauses, thus a CWA clause could be presented for some predicates and not for others. We will define an argument as a subset of a dlp, and therefore we can use some concepts of an argumentative system [12] in order to define the inference engine of the system. The semantics of a dlp is characterized by sets of positive, negative, undecided and unknown answers. Finally, we will define the behavior of an interpreter based upon this semantics. 1 Published in proceedings of the XXI Latin American Conference of Informatic, Brasil, 1995. 2 This work was partially supported by the Secretar´ ıa de Ciencia y T´ ecnica, Universidad Nacional del Sur. 3 Fellow of Consejo Nacional de Investigaciones Cient´ ıficas y T´ ecnicas (CONICET), Rep´ ublica Argentina