Or, more cautiously (sometimes I’m cautious), I find defeat talk misleading. And here’s why.
Let q be the true proposition that S gives testimony with content p. This is a reason for me, at t1, the time of my hearing the testimony, to accept p, since I’m aware that S is generally reliable. But then I learn, at t2, d: that in spite of his general reliability, S is known to be unreliable when it comes to p-type matters.
In defeat language, d defeats the support q gives to p. But what can this mean? What has been defeated? Certainly not the connection between my original evidence (captured in E1 below) and p. Consider the following two sets of evidence.
E1: S said “p” & S is generally reliable.
E2: E1 & S is unreliable concerning p-type questions.
E1 characterizes my evidence at t1, and E2 characterizes my evidence at t2. At t1 my total relevant evidence supported p. At t2 it did not. But what is it that has been defeated? Not my justification at t1! Justification is a synchronic matter. My transition from being justified in believing p to not being so is a diachronic matter.
And even at t2 E1 supports p. Its support for p is an objective relation it bears at all times if at any time. That relation doesn’t fail to bear just because it’s part of a larger body of evidence such that that larger body of evidence doesn’t bear that relation.
In short: If there is defeat, then there is something that is defeated. But there is no viable candidate for anything that has been defeated.
I followed Chisholm in defining defeat over a support relation (3rd ed, 55). But I don’t see that it’s any better following Bergmann in defining defeat over target beliefs or propositions. My justification at t1 was never defeated. I just got new evidence and on that evidence I was no longer justified. Nothing is defeated.
We sometimes talk about one item of information “screening off” another, but I think that’s more helpful when thinking about causation. Perhaps defeat talk is innocuous, but I find it misleading and sometimes (today) it irks me.