 The argument requires a slight extension of our epistemic logic, to accommodate the notion of knowability. For us, \(K\) means known (or entailed by the known), whereas knowability adds an extra modal layer: what it’s possible to know. So we’ll need a sentential operator \(\Diamond\) in our language to represent metaphysical possibility. Thus \(\Diamond \phi\) means “it’s metaphysically possible for \(\phi\) to be true”. In fact, \(\Diamond \phi\) is just short for \(\neg \Box \neg \phi\), since what doesn’t have to be false can be true. So we can actually add the \(\Box\) instead and assume that, like the \(K\) operator, it obeys the NEC rule. (As with the NEC rule for the \(K\) operator, it’s okay that we can always derive \(\Box \phi\) from \(\phi\), because we can only derive \(\phi\) in the first place when \(\phi\) is a logical truth.) \(\Diamond\) is then just \(\neg \Box \neg\) by definition.