A defeater of a

belief A belief is an attitude that something is the case, or that some proposition is true. In epistemology, philosophers use the term "belief" to refer to attitudes about the world which can be either true or false. To believe something is to tak ...

evidence Evidence for a proposition is what supports this proposition. It is usually understood as an indication that the supported proposition is true. What role evidence plays and how it is conceived varies from field to field. In epistemology, evidenc ...

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

epistemology Epistemology (; ), or the theory of knowledge, is the branch of philosophy concerned with knowledge. Epistemology is considered a major subfield of philosophy, along with other major subfields such as ethics, logic, and metaphysics. Epi ...

since 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 philosophical logic, defeasible reasoning is a kind of reasoning that is rationally compelling, though not deductive reasoning, deductively valid. It usually occurs when a rule is given, but there may be specific exceptions to the rule, or su ...

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 mental process of drawing deductive inferences. An inference is deductively valid if its conclusion follows logically from its premises, i.e. if it is impossible for the premises to be true and the conclusion to be fal ...

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''.

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