Defeater

A defeater, in philosophy and particularly epistemology, refers to evidence or a piece of information that undermines the justification for a belief. It weakens or neutralizes the reasons one has for holding a belief, thereby potentially rendering the belief unjustified, even if the belief happens to be true. Defeaters don't necessarily prove a belief is false, but they do show that the reasons for holding the belief are unreliable or insufficient.

Defeaters are typically categorized into two main types:

• Undercutting Defeaters: These attack the connection between the evidence and the belief. An undercutting defeater doesn't directly contradict the belief itself, but instead casts doubt on the reliability or validity of the evidence used to support the belief. For instance, believing you saw a friend across the street, but then learning you recently took a medication that causes visual hallucinations. The medication doesn't prove your friend wasn't there, but it undermines your confidence in your visual perception as evidence.

• Rebutting Defeaters: These directly contradict the belief itself. A rebutting defeater provides evidence that the belief is actually false. For example, believing a friend is home because their car is in the driveway, but then receiving a text message from them saying they are on vacation in another country. The text message is direct evidence contradicting the belief that they are home.

The presence of a defeater doesn't automatically make a belief irrational to hold. The strength and nature of the defeater must be considered. One may still be justified in holding a belief if the defeater is weak or if there are other, stronger reasons to maintain the belief. The process of responding to defeaters, often involving gathering more evidence or revising one's reasoning, is a crucial aspect of responsible belief formation and maintenance.

The concept of defeaters is also relevant in artificial intelligence and formal argumentation theory, where they are used to model reasoning under uncertainty and handle conflicting information.