Argumentation is the activity of producing explicit justifications for the assertions of a reasoner. It plays a crucial role in a debate, both in revising one's own assertions and in comparing and opposing them to assertions of other reasoners. Several recent works have provided detailed logical characterizations of argument properties, paying however only limited attention to the conceptual structure of an argument, which is indeed a fundamental issue for a correct modeling of the argumentation activity in practical contexts. Starting from the concept of argument life cycle, we propose in this paper a model of argumentation activity encompassing the main phases of argument construction, criticism, and corroboration. Then we introduce a structured representation of an argument, based on the concept of justification graph, and we use it as a basis for the analysis of argument life cycle phases. A simple argumentation example, concerning an athlete doping case, is developed throughout the paper in order to support an intuitive understanding of the proposed ideas.
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