 We write \(\alpha \vDash \beta \) if for all valuations \(V\), \(V(\alpha) \subseteq V(\beta).\) Then, \( \alpha \vDash \beta\) expresses the fact that no matter how the non-logical vocabulary of \(\mathbf{L}\) are interpreted, \(\beta\) is true in all the worlds in which all sentences in \(\alpha\) are true. We say that \(\alpha\) is  valid iff \(\{ \top \} \vDash \alpha\), i.e if \(W\subseteq V(\alpha)\) for all valuation functions. Then, \(\alpha\) is valid iff \(\alpha\) is true in all possible worlds, no matter how the non-logical vocabulary are interpreted. For example, the sentence \(\alpha\vee \neg \alpha\) is valid.