On November 5th, 2024, our colleague and friend Kees Doets passed away peacefully and in full possession of his mental faculties, ending a long period of struggling with serious health issues
Kees Doets was born in Vlaardingen on 3 January 1941 and passed away on 5 November 2024 in Amstelveen. He studied mathematics at the University of Amsterdam where he obtained his master's degree with Arend Heyting in 1966. In 1967 he was appointed as a young assistant professor in logic in the philosophy department, starting 35 years of service in the department of philosophy, later that of mathematics, and eventually the Institute for Logic, Language and Computation, until his retirement in 2002.
Kees Doets was an exceptionally gifted teacher, combining compact presentation with clarity and precision, influencing many generations of students who took his courses Introduction to Mathematical Logic, Model Theory, and Set Theory. These virtues can also be observed in the remarkably well-written 1975 textbook "Verzamelingen: Naïef, Axiomatisch en Toegepast" with co-authors Dirk van Dalen and Harrie de Swart. Much later, his course on model theory would inspire his elegant little textbook "Basic Model Theory" from 1996, published at Stanford University, which is still a gateway to the field for beginning students.
Kees' research interests and talents in logic were on the model-theoretic and set-theoretic side, making him a valuable outsider in a long constructivism-dominated Amsterdam logic environment. He put these talents to work in an important chapter on 'Higher-Order Logic' published in the "Handbook of Philosophical Logic" of 1983, infusing the somewhat scattered field of higher-order logic with a systematic perspective and a clarity that explains its continued use as a reference source. Further research, much of it involving his beloved 'Ehrenfeucht-Fraïssé games' for comparing models as to their first-order definable properties, led to important results with Doets-style concise and elegant proofs on axiomatizing higher-order theories of well-founded orders and applications to modal logics. This research was collected in his 1987 dissertation "Completeness and Definability. Applications of the Ehrenfeucht Game in Second-Order and Intensional Logic", supervised by his student and friend Johan van Benthem.
In subsequent years, Kees turned to the theory of Logic Programming, a new phase of his teaching and research which culminated in his 1994 book "From Logic to Logic Programming", half of a diptych with his colleague and friend Krzysztof Apt's more computationally oriented book "From Logic Programming to Prolog". In 2004 a cooperation of Kees with Jan van Eijck resulted in one more book, ``The Haskell Road to Logic, Maths and Programming'' that soon became one of the most successful introductions tothe Haskell programming language.
Kees Doets was no stranger to academic service, both in Amsterdam and abroad, inside logic and beyond. In particular, he served as a secretary to the Program Committee of the World Congress of Logic, Methodology and Philosophy of Science, becoming one of the four editors of the resulting "Proceedings 10th International Congress on Logic, Methodology and Philosophy of Science. Florence 1995 I (Logic and Scientific Methods), II (Structures and Norms in Science)".
Kees retired from the University of Amsterdam in 2002, remaining in touch about shared interests with his closest former colleagues and friends. Also, till the end of his life Kees keenly read the weekly ILLC News and commented on selected items that caught his attention. But perhaps most of all, Kees kept actively pursuing the various physical sporting interests that had characterized his whole life.
Kees Doets will be greatly missed by his colleagues, former students, and friends who all highly appreciated his dedication to teaching and high quality research and who all admired his modesty, charm and warmth. Kees leaves behind four children and grandchildren.
A small memorial website with personal statements from Kees Doets' friends and colleagues at the ILLC and a list of his publications may be found at https://resources.illc.uva.nl/emeriti/Kees-Doets/.