Defeasible Reasoning

Defeasible reasoning is inference that is valid in the absence of defeating information but becomes invalid when new evidence emerges. Unlike deductive reasoning, where conclusions follow necessarily from premises, defeasible reasoning produces conclusions that are provisionally justified — held as 'accurately or very nearly true,' in Newton's phrase — until contrary phenomena appear. The concept is central to epistemology, artificial intelligence, and legal reasoning, where agents must act on incomplete information while remaining prepared to revise their commitments.

The philosophical significance of defeasible reasoning is that it captures how actual minds — and actual scientific communities — operate. No empirical inference is ever final; every generalization is a standing invitation to counterexample. The architecture of defeasible reasoning connects to non-monotonic logic in AI and to belief revision theory in formal epistemology, both of which attempt to model how rational agents should update their beliefs when new information conflicts with old conclusions.