A belief which, if proved to be true, would imply outright or indirectly that another belief were false(noun)
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Use "defeater" in a sentence
"Once I see the connection, then I have a defeater: the defeater is the conjunction of those propositions, together with the proposition that if that conjunction is true, then you weren't at the game at 9: 30."
"Second, the kind of defeater allegedly provided is undercutting, not rebutting."
"The point is only that the explanatory idleness of my belief does not constitute any kind of defeater for it -- because it isn't accepted as an explanation."