A defeater of a

belief A belief is a subjective Attitude (psychology), attitude that something is truth, true or a State of affairs (philosophy), state of affairs is the case. A subjective attitude is a mental state of having some Life stance, stance, take, or opinion ...

evidence Evidence for a proposition is what supports the proposition. It is usually understood as an indication that the proposition is truth, true. The exact definition and role of evidence vary across different fields. In epistemology, evidence is what J ...

that this belief is false. Defeaters are of particular interest to

epistemology Epistemology is the branch of philosophy that examines the nature, origin, and limits of knowledge. Also called "the theory of knowledge", it explores different types of knowledge, such as propositional knowledge about facts, practical knowle ...

because they affect whether a belief is justified. An important distinction is between undercutting and rebutting defeaters. Undercutting defeaters remove evidential support for a belief while rebutting defeaters provide evidential support for the opposite thesis of the belief. Defeaters play a central role in modern developments of

defeasible reasoning In philosophy of logic, defeasible reasoning is a kind of provisional reasoning that is rationally compelling, though not deductively valid. It usually occurs when a rule is given, but there may be specific exceptions to the rule, or subclasse ...

Types

There are two types of defeaters: ''rebutting defeaters'' and ''undercutting defeaters''.

Rebutting

Evidence for the opposite thesis of a belief is called a rebutting defeater of this belief. For example, looking through the window and seeing that the sky is clear is evidence for the belief that it is not raining outside. Therefore, this perception is a rebutting defeater of the belief that it is raining.

Undercutting

Evidence that undermines the evidential support for a belief without giving support to the opposite thesis is called an undercutting defeater of this belief. For example, remembering that one just consumed a psychedelic drug is evidence against the belief that it is not raining. This memory undermines the reliability of the perception of the clear sky on which the belief was based. But at the same time it doesn't give evidential support to the opposite belief that it is raining. This memory is, therefore, an undercutting defeater of the belief that it is raining.

Role in defeasible reasoning

Defeaters play a central role in modern developments of

. In traditional

deductive reasoning Deductive reasoning is the process of drawing valid inferences. An inference is valid if its conclusion follows logically from its premises, meaning that it is impossible for the premises to be true and the conclusion to be false. For example, t ...

the only way the conclusion of a valid argument can be false is if at least one of the premises is false. A defeasible argument, on the other hand, allows the retraction of its conclusion as new evidence is acquired without denying the truth of its premises. The evidence responsible for this retraction is called a ''defeater''.

References

External links