Defeasibility in Legal Reasoning*

This chapter provides a logical analysis of defeasible reasoning. First it introduces the notion of a reasoning schema, and considers how ratiocination proceeds according to the reasoning schemata adopted by the concerned agent. It then distinguishes conclusive and defeasible reasoning, and argues that cognitive agents need to engage in defeasible reasoning for coping with a complex and changing environment. It analyzes the process of defeasible reasoning considering collisions of reasons, defeat, preference-based reasoning and reinstatement, as well as the distinction between rebutting and undercutting. Finally, it considers defeasibility in practical and legal reasoning, and argues that defeasible reasoning constitutes an essential aspect of legal problem-solving.