I see your point and I think we mostly agree about everything. My only slight extra point is to suggest that perhaps Warren was trying to use his prior beliefs to predict an explanation for absence of sabotage, rather than trying to use absence of sabotage to intensify his prior beliefs. In retrospect, it’s likely that you’re right about Warren and the quote makes it seem that he did, in fact, think that absence of sabotage increased likelihood of Fifth Column. But in general, though, I think a lot of people make a mistake that has more to do with starting out with an unreasonable prior, or making assumptions that their prior belief is independent of observations, than it has to do with a logical fallacy about letting conditioning on both A and ~A increase the probability of B.
I see your point and I think we mostly agree about everything. My only slight extra point is to suggest that perhaps Warren was trying to use his prior beliefs to predict an explanation for absence of sabotage, rather than trying to use absence of sabotage to intensify his prior beliefs. In retrospect, it’s likely that you’re right about Warren and the quote makes it seem that he did, in fact, think that absence of sabotage increased likelihood of Fifth Column. But in general, though, I think a lot of people make a mistake that has more to do with starting out with an unreasonable prior, or making assumptions that their prior belief is independent of observations, than it has to do with a logical fallacy about letting conditioning on both A and ~A increase the probability of B.