  Many disagreements are one-on-one: one person disagrees with another person and as far as they know they are the only two who have any opinion on the matter. Lisa thinks that she and Marie should move in together; then Lisa discovers that Marie has the opposite opinion. Bob and his sister Teri disagree about whether their father had an affair when they were children. In this case they know that others have the answer—their father, for one—but for various reasons the opinions of others are not accessible.