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Goal Selection in Argumentation Processes - A Formal Model of Abduction in Argument Evaluation Structures

When argumentation is conceived as a kind of process, typically a dialogue, for reasoning rationally with limited resources under conditions of incomplete and inconsistent information, arguers need heuristics for controlling the search for arguments to put foward, so as to move from stage to stage in the process in an efficient, goal-directed way. For this purpose, we have developed a formal model of abduction in argument evalution structures. An argument evaluation structure consists of the arguments of a stage, assumptions about audience and an assignment of proof standards to issues. A derivability relation is defined over argument evaluation structures for the literals 'in' a stage. Literals which are not derivable in a stage are 'out'. Abduction is defined as a relation between an argument evaluation structure and sets of literals, called 'positions', which, when the assumptions are revised to include the literals of the position, would make a goal literal in or out, depending of the standpoint of the agent. Soundness, minimiality, consistency and completeness properties of the abduction relation are proven. A heuristic cost function estimating how difficult it is to find or construct arguments pro a literal in the domain can be used to order positions and literals within positions. We compare our work to abduction in propositional logic, in particular the Assumption-Based Truth Maintenance System (ATMS).