Fitchean Ignorance and First-order Ignorance

A Neighborhood Look

Abstract

In a seminal work Fine (2018), Fine classifies several forms of ignorance, among which are Fitchean ignorance, first-order ignorance, Rumsfeld ignorance, and second-order ignorance. It is shown that there are some relationships among some of them, including that in S4, all higher-order ignorance are reducible to second-order ignorance. This is thought of as a disadvantage by some researchers. It is then natural to ask how to avoid this consequence. We deal with this issue in a much more general framework. In details, we treat the forms of Fitchean ignorance and first-order ignorance as primitive modalities and study them as first-class citizens under neighborhood semantics, in which Rumsfeld ignorance and second-order ignorance are definable. The main contributions include model-theoretical results such as expressivity and frame definability, and axiomatizability. Last but not least, by updating the neighborhood models via the intersection semantics, we extend the results to the dynamic case of public announcements, which gives us some applications to successful formulas. We also show that the result in Fine (2018) stating all higher-order ignorance are reducible to second-order ignorance does not hold in any of our logical systems.