IN MEMORIAM: J. MICHAEL DUNN, 1941–2021

In 1969, Dunn was appointed an associate professor in the Department of Philosophy at Indiana University in Bloomington, Indiana, and he stayed on the faculty at IU until 2007, when he retired as University Dean of the School of Informatics, Oscar R. Ewing Professor of Philosophy, Professor of Informatics, Professor of Computer Science, and Core Faculty in Cognitive Science. Dunn showed that $\boldsymbol{\mathit{4}}$ , the four-element lattice with two incomparable elements on which negation has fixed points, plays a fundamental role among De Morgan lattices, and hence, for first-degree entailments fde; (the implication-free fragment of $\mathbf {R}$ and of the logic of entailment $\mathbf {E}$ ). A simple form of this incompleteness is practically obvious to anybody who is familiar with standard definitions of the language and the interpretation of fol; however, [36] proved incompleteness in a stronger sense using Gödel’s incompleteness theorem. To wit, logical consequence that is invariant under extensions of the language by new name constants circumvents the crucial step in the incompleteness argument; this notion is pivotal in Henkin-style completeness proofs for first-order logics.