Preface: In memory of Wim Blok

algebraic logic: Full models, Frege systems, and metalogical properties he formulates an institutional analogue of the property of congruence and analyses how it helps in the preservation of other metalogical properties such as conjunction, disjunction, the deduction-detachment theorem, and two versions of reductio ad absurdum. In partial contrast, Raftery’s paper The equational definability of truth predicates explores a new direction in the study of the Leibniz hierarchy without leaving its original framework, that is, the algebraic study of deductive systems of sentential character. Most of Wim’s work in abstract algebraic logic concerned the largest class in the hierarchy, namely the class of protoalgebraic logics, which he and Don introduced in [2]. One of the several different ways in which this class is characterized is by the property that the Lebniz operator is monotonic on the lattice of theories of the logic. Initially it was believed that this class encompassed all logics whose matrix semantics was amenable to the lattice-theoretic and model-theoretic methods of traditional algebraic logic and of the more modern universalAlready in Appendix C of [3] the authors show how to apply the method to account for the algebraization of first-order logic in terms of cylindric algebras.