 A further proposal that uses knowledge as the basic notion is that of Baltag, Bezhanishvili, Özgün, and Smets (2013), which generalizes Stalnaker’s (2006) formalization by using a topological (neighborhood) semantics. An important feature of the notion of belief that arises in this setting is that it is subjectively indistinguishable from knowledge: an agent believes \(\varphi\) \((B\varphi)\) if and only if she believes that she knows it \((BK\varphi)\).