  However, \(\mathbf{S5}\) is not a reasonable logic for all members of the modal family. In deontic logic, temporal logic, and others, the analog of the truth condition (5) is clearly not appropriate; furthermore there are even conceptions of necessity where (5) should be rejected as well. The point is easiest to see in the case of temporal logic. Here, the members of \(W\) are moments of time, or worlds “frozen”, as it were, at an instant. For simplicity let us consider a future temporal logic, a logic where \(\Box A\) reads: ‘it will always be the case that’. (We formulate the system using \(\Box\) rather than the traditional \(G\) so that the connections with other modal logics will be easier to appreciate.) The correct clause for \(\Box\) should say that \(\Box A\) is true at time \(w\) iff \(A\) is true at all times in the future of \(w\). To restrict attention to the future, the relation \(R\) (for ‘earlier than’) needs to be introduced. Then the correct clause can be formulated as follows.