Jake Chandler in his paper “Defeat Reconsidered” (2013) developed the following counterexample: Outside the door to Sam’s flat is a switch for the light in the staircase. Flipping the switch (ES) typically causes the light to go on (PS): ES is a reason to believe PS. When there is a power cut (DS), ES loses this probative force. Thus, DS is a defeater for ES’s support for PS. It is also part of DS that there is backup power system that is activated (and automatically turns on the lights) when the main system fails. So, just like ES, DS provides a reason to believe PS. But there is an asymmetry: while DS is a defeater for ES’s support for PS, ES is not a defeater for DS’s support for PS (since the position of the switch is irrelevant).
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