 In modal logic, a Kripke frame is a pair \(\langle W, R\rangle\) where \(W\) is a non-empty set of possible worlds (possible states, possible situations or possible ways the world might be) and \(R\) is a binary relation on \(W\). \(R\) is called the accessibility relation of the model, and intuitively \(Rwv\) means that world \(v\) is (in some sense) possible from the point of view of \(w\). A model is a triple \(\langle W, R, V\rangle\) such that \(\langle W, R\rangle\) is a frame and \(V\) is a valuation function assigning truth values to all propositional variables of the language at all possible worlds in \(W\). We write \(M, w \Vdash A\) for “\(A\) is true at world \(w\) in model \(M\)” and let \(\lvert A\rvert^M\) denote the set \(\{w\in W: M, w \Vdash A\}\) of possible worlds at which \(A\) is true. Let \(R(w) := \{v\in W: Rwv\}\). The truth conditions for the Boolean connectives are the usual ones for propositional logic, extending \(V\) to complex sentences. Those for the strict conditional are as follows: