In “a paper in today’s issue of Philosophical Quarterly”:http://www.ingentaconnect.com/content/bpl/phiq/2005/00000055/00000220/art00003, Michael Bergmann has a discussion of defeaters. Here are the definitions he provides for rebutting and undercutting defeaters.
* d is a rebutting defeater for b iff d is a defeater for b which is (or is an
epistemically appropriate basis for) the belief that b is false
* d is an undercutting defeater for b iff d is a defeater for b which is (or is an
epistemically appropriate basis for) the belief that ones actual ground or
reason for b is not indicative of bs truth.
It seems to me these definitions can’t really get at what was driving the identification of these classes. Two examples about chance show this.
Let d be the belief that the objective chance of b is, right now, 0.2. It seems to me that is a _rebutting_ defeater. But since it isn’t sufficient grounds to conclude that b is false, it has to be taken to be an undercutting defeater. And that’s so even if d is not in any sense about my epistemic practices that led me to believe that b.
Second example. Let d be the belief that the objective chance of b is, right now, 0.8. In some circumstances, that is incompatible with believing that b, so it is a defeater. But it is clearly not a rebutting defeater. And, since it is compatible with thinking the processes that led to belief that b are very reliable, it isn’t really an undercutting defeater either. So on this definition rebutting and undercutting defeaters aren’t exhaustive, which seems bad.