  A formula is true at a context \(c\) in a structure \(M\) iff: the formula is true with respect to \(c\) and the time and world of \(c\) in \(M\). A formula is valid iff: for every structure \(M\), and every context \(c\) of \(M\), the formula is true at \(c\) in \(M\). Kaplan says that the notion of logical consequence can be defined in any of the usual ways.