The pivotal characteristic of erotetic calculi, as introduced by Wiśniewski, is that its construction serves both purely proof-theoretical purposes and more philosophical—erotetic—aims. Although this book focuses on proof-theoretical aspects of the method, the picture would certainly not be complete without the analysis of questions. The idea standing behind the method of Socratic proofs was to model reasoning in which a question concerning derivability in the underlying logic is answered by ‘pure questioning’, without the reference to an external source of information (see, first of all, the introductions to [21, 26]). Therefore, the construction of ‘Socratic’ calculi consists in a formalization of rules transforming questions concerning derivability in the underlying logic. One could ask: what is meant by ‘derivability’ here, but we shall not make an attempt to answer this question until Sect. 5.6. At the time being, we assume that some common intuitions concerning the notion are sufficient.