 Figure. Consider a universe of 4 worlds. If we are not in fact brains-in-a-vat (so that the hypothesis \(v\) that we are brains in a vat is false, \(\neg v\) is true at \(w^*\), the actual world) we can nonetheless entertain \((E)\), the hypothesis that we are (so that \(Ev\) is true at \(w^*\)), recognizing as we do so that were we to inhabit a world such as \(w'\) in which we are brains-in-a-vat (\(v\) holds at \(w'\)), we would lack the semantic resources to articulate thoughts reflecting our own envatted state so that we could not so much as entertain the thought that we were brains-in-a-vat (\(\neg Ev\) holds not only at \(w'\) but at all those worlds ( \(w''\), \(w'''\)) accessible from \(w'\)).