defeaters

Defeaters are propositions, arguments, or rules that undermine, counter, or prevent the acceptance of a claim or inference. In broad terms, a defeater weakens or defeats a conclusion that would otherwise be supported by available evidence, reasoning, or rules. The concept is central to theories that allow conclusions to be provisional or revisable in light of new information.

In epistemology and related fields, a defeater is information that defeats the justification for a belief. A belief may be justified given current evidence, but the introduction of a defeater—such as a reliable counterexample or an argument that undermines the relevance of the evidence—can render the belief unwarranted. Defeaters help explain why rational agents revise their beliefs when confronted with new data.

In defeasible reasoning and many artificial intelligence frameworks, conclusions are typically defeasible: they hold in the absence of defeaters but can be withdrawn when contrary information becomes available. Defeaters in this setting are often classified as rebutting defeaters, which directly contradict the conclusion, and undercutting defeaters, which attack the link between the premises and the conclusion (the warrant or rule used to infer the conclusion).

In argumentation theory and related disciplines, a defeater corresponds to a counterargument or piece of evidence that defeats another argument's force. It is possible for multiple defeaters to interact, leading to the rejection of some conclusions and the reinforcement of others, depending on the strength and relevance of the competing information.

Overall, defeaters describe mechanisms by which beliefs or conclusions can be challenged or overturned, reflecting the dynamic nature of reasoning under uncertainty.