In one of his comments on testimony and defeat, Jon K writes, “My view on defeaters is that if you think you have one, you do.” This is not an uncommon position, as Michael Bergmann shows in some of his papers on defeat. He endorses a “no defeater” clause on justification that says something to the effect that, if S believes that her belief that p is defeated, then S is not justified in believing that p, and he cites a number of externalist epistemologists saying about the same thing.
This is usually in response to counterexamples like those made famous by BonJour. In his clairvoyant examples, S’s belief that p is in fact reliably formed, but S has lots of other evidence against the belief that p, or against the belief that the belief that p was reliably formed. The typical externalist response is to say that the beleif is not justified because S has a defeater for it.
This strikes me as odd. To put it roughly, it seems like one is offering an externalist account of positive or supporting evidence for a beleif, but then “going internalist” about counterevidence or defeating evidence. Why should an externalist think that believing you have counterevidence makes it true that you have counterevidence? That is very close to endorsing the following principle: Jp => not-B(not-Jp). Which is equivalent to: B(not-Jp) => not-Jp. But what self-respecting externalist would say that?
If anyone is intersted, I am working on a paper about how externalists should think about defeat. I am working on the paper now, so any comments would be welcome.