 Some readers may wonder, why do I need to know whether Ann believes that Bob’s assumption is wrong? One of the reasons is that, just as  Russell’s paradox  suggests that not every collection can constitute a set, this situation, known as the Brandenburger-Keisler Paradox (Brandenburger &amp; Keisler 2006), suggests that not every description of beliefs can be ‘represented’. Now, with a better motivation, one may wonder again: is \(\varphi\) true or false? Or, maybe better, is there a formal setting that gives an answer?