Argument schemes for two-phase democratic deliberation

A formal two-phase model of democratic policy deliberation is presented, in which in the first phase sufficient and necessary criteria for proposals to be accepted are determined (the `acceptable' criteria) and in the second phase proposals are made and evaluated in light of the acceptable criteria resulting from the first phase. Such a separation gives the discussion a clear structure and prevents time and resources from being wasted on evaluating arguments for proposals based on unacceptable criteria. Argument schemes for both phases are defined and formalised in a logical framework for structured argumentation. The process of deliberation is abstracted from and it is assumed that both deliberation phases result in a set of arguments and attack and defeat relations between them. The acceptability status of criteria and proposals within the resulting argumentation framework is then evaluated using preferred semantics. For cases where preferences are required to choose between proposals, inference rules for deriving preferences between sets from an ordering of their elements are given.