  Christensen (2007) responds to this challenge by noting that while merely possible disagreement only shows that we are fallible, actual disagreement demonstrates that someone has in fact made a mistake. Since we are already aware that we are fallible epistemic agents, thinking about possible peer disagreements does not add any information that calls for (further) doxastic change. In contrast, discovering an actual peer disagreement gives us information that we lacked. In a case of peer disagreement, one of the parties has made a mistake. While the possibility of error does not demand belief revision, an increase in the probability of having made an error does.