 Note how this is exactly what the definition of belief in the previously discussed plausibility models entails: if the modality \([\leq]\) is understood as indefeasible knowledge (Lehrer 1990; Lehrer &amp; Paxson 1969), then \(B\varphi := \langle{\leq}\rangle[{\leq}]\varphi\) states that belief is the possibility of knowledge. In this context, it is a small step to move from studying simple beliefs to conditional beliefs. A complete axiomatization of the logic of indefeasible knowledge and conditional belief, first posed as an open question in Board (2004), was provided with a solution in Baltag and Smets (2008).