 \(\Box\psi\) is true if and only if \(\psi\) is true at all states of affairs \(w\) such that (i) for all states of affairs \(s,\) either, necessarily, \(w\) obtains only if \(s\) does or, necessarily, \(w\) obtains only if \(s\) doesn’t, and (ii) possibly, \(w\) obtains.