 A plausibility model is a relational model \(M = {\langle W, \leq, V \rangle}\) in which the binary relation \(\leq\) is interpreted as describing the plausibility ordering the agent assigns to her epistemic possibilities \((w \leq u\) indicates that, for the agent, world w is at least as plausible as world u). In the single-agent case, the plausibility relation \(\leq\) is required to be a well-preorder: a total relation which is both reflexive and transitive, and such that every non-empty subset of the domain has \(\leq\)-minimal elements. These minimal elements in W are then understood as the agent’s most plausible worlds. We see below that what is true in all the most plausible worlds characterizes what an agent believes.