[There] is . . . a problem that bedevils all of us as members of communities of believers. I call this problem our disagreement deficit, and it comes in four parts. . . . First, our communities expose us to disproportionate support for our own ideas. Second, they shield us from the disagreement of outsiders. Third, they cause us to disregard whatever outside disagreement we do encounter. Finally, they quash the development of disagreement from within.