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<title>Defeater</title>

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<span id="openzim-page-title" class="mw-page-title-main"><span class="mw-page-title-main">Defeater</span></span>

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</style><div role="note" class="hatnote navigation-not-searchable">For the hardcore punk band, see <a href="Defeater_(band)" title="Defeater (band)">Defeater (band)</a>.</div>

<p>A <b>defeater</b> of a <a href="Belief" title="Belief">belief</a> is <a href="Evidence" title="Evidence">evidence</a> that this belief is false.<sup id="cite_ref-1" class="reference"><a href="#cite_note-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> Defeaters are of particular interest to <a href="Epistemology" title="Epistemology">epistemology</a> because they affect whether a belief is <a href="Justification_(epistemology)" title="Justification (epistemology)">justified</a>. 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 <a href="Defeasible_reasoning" title="Defeasible reasoning">defeasible reasoning</a>.

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<div class="mw-heading mw-heading2"><h2 id="Types">Types</h2></div>

<p>There are two types of defeaters: <i>rebutting defeaters</i> and <i>undercutting defeaters</i>.<sup id="cite_ref-2" class="reference"><a href="#cite_note-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading3"><h3 id="Rebutting">Rebutting</h3></div>

<p>Evidence for the opposite thesis of a belief is called a rebutting defeater of this belief.<sup id="cite_ref-Stanford_3-0" class="reference"><a href="#cite_note-Stanford-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> 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.<sup id="cite_ref-Lee_4-0" class="reference"><a href="#cite_note-Lee-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading3"><h3 id="Undercutting">Undercutting</h3></div>

<p>Evidence that undermines the evidential support for a belief without giving support to the opposite thesis is called an undercutting defeater of this belief.<sup id="cite_ref-Stanford_3-1" class="reference"><a href="#cite_note-Stanford-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> 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.<sup id="cite_ref-Lee_4-1" class="reference"><a href="#cite_note-Lee-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup>

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<div class="mw-heading mw-heading2"><h2 id="Role_in_defeasible_reasoning">Role in defeasible reasoning</h2></div>

<p>Defeaters play a central role in modern developments of <a href="Defeasible_reasoning" title="Defeasible reasoning">defeasible reasoning</a>. In traditional <a href="Deductive_reasoning" title="Deductive reasoning">deductive reasoning</a> the only way the conclusion of a valid argument can be false is if at least one of the premises is false.

</p><p>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 <i>defeater</i>.

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<ul><li><a href="Argumentation_framework" title="Argumentation framework">Argumentation framework</a></li>

<li><a href="Inference_objection" class="mw-redirect" title="Inference objection">Inference objection</a></li></ul>

<div class="mw-heading mw-heading2"><h2 id="References">References</h2></div>

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</style><cite id="CITEREFMcCainStaplefordSteup2021" class="citation book cs1">McCain, Kevin; Stapleford, Scott; Steup, Matthias (21 October 2021). <a rel="nofollow" class="external text" href="https://books.google.com/books?id=IeNBEAAAQBAJ&pg=PT111"><i>Epistemic Dilemmas: New Arguments, New Angles</i></a>. Routledge. p. 111. <a href="ISBN_(identifier)" class="mw-redirect" title="ISBN (identifier)">ISBN</a> <bdi>978-1-000-46851-9</bdi>.</cite></span>

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<li id="cite_note-2"><span class="mw-cite-backlink"><b><a href="#cite_ref-2">^</a></b></span> <span class="reference-text"><cite id="CITEREFPollock1987" class="citation journal cs1">Pollock, John L. (1987). <span class="id-lock-subscription" title="Paid subscription required"><a rel="nofollow" class="external text" href="https://onlinelibrary.wiley.com/doi/abs/10.1207/s15516709cog1104_4">"Defeasible Reasoning"</a></span>. <i>Cognitive Science</i>. <b>11</b> (4): <span class="nowrap">481–</span>518. <a href="Doi_(identifier)" class="mw-redirect" title="Doi (identifier)">doi</a>:<span class="id-lock-free" title="Freely accessible"><a rel="nofollow" class="external text" href="https://doi.org/10.1207%2Fs15516709cog1104_4">10.1207/s15516709cog1104_4</a></span>. <a href="ISSN_(identifier)" class="mw-redirect" title="ISSN (identifier)">ISSN</a> <a rel="nofollow" class="external text" href="https://search.worldcat.org/issn/1551-6709">1551-6709</a>.</cite></span>

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<li id="cite_note-Stanford-3"><span class="mw-cite-backlink">^ <a href="#cite_ref-Stanford_3-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Stanford_3-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><cite id="CITEREFKelly2006" class="citation web cs1">Kelly, Thomas (11 August 2006). <a rel="nofollow" class="external text" href="https://plato.stanford.edu/archives/win2016/entries/evidence/">"Evidence"</a>. <i>The Stanford Encyclopedia of Philosophy</i>.</cite></span>

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<li id="cite_note-Lee-4"><span class="mw-cite-backlink">^ <a href="#cite_ref-Lee_4-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-Lee_4-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><cite id="CITEREFLee2017" class="citation thesis cs1">Lee, James Soo (August 2017). <a rel="nofollow" class="external text" href="https://surface.syr.edu/etd/765">"I. Introduction"</a>. <i>A Metaphysician's User Guide: The Epistemology of Metaphysics</i> (PhD thesis). Syracuse, NY: <a href="Syracuse_University" title="Syracuse University">Syracuse University</a>.</cite></span>

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<div class="mw-heading mw-heading2"><h2 id="External_links">External links</h2></div>

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<div class="side-box-text plainlist">Look up <i><b><a href="https://en.wiktionary.org/wiki/Special:Search/defeater" class="extiw external" title="wiktionary:Special:Search/defeater">defeater</a></b></i> in Wiktionary, the free dictionary.</div></div>

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<ul><li><a rel="nofollow" class="external text" href="https://philpapers.org/browse/defeat">Defeater</a> at <a href="PhilPapers" title="PhilPapers">PhilPapers</a></li>

<li><cite class="citation encyclopaedia cs1"><a rel="nofollow" class="external text" href="https://iep.utm.edu/ep-defea">"Defeaters in Epistemology"</a>. <i><a href="Internet_Encyclopedia_of_Philosophy" title="Internet Encyclopedia of Philosophy">Internet Encyclopedia of Philosophy</a></i>.</cite></li>

<li><a rel="nofollow" class="external text" href="https://plato.stanford.edu/entries/reasoning-defeasible/">"Defeasible Reasoning"</a> entry in the <i><a href="Stanford_Encyclopedia_of_Philosophy" title="Stanford Encyclopedia of Philosophy">Stanford Encyclopedia of Philosophy</a></i></li></ul></div><div><div class="zim-footer">

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