In Memoriam – Krister Segerberg

A word of fond memory about Krister

I am the editor of David Kaplan’s collected writings and this is from a meeting of ours. David was much touched by Krister's passing, he knew him well and for more than 45 years.

David recalls Krister as Dana's student in Stanford late mid sixties, it is somehow mixed in memory with his own JSL review of 1966 (and interactions with Scott and Lemmon who was in southern California). In the JSL 1966 review a generalized completeness for modal logics is conjectured. David recalls how Krister, on a later visit of David to Stanford (Stanford-UCLA had regular joint activities in those Montague-Scott times) conjectured to David that this JSL conjecture is going to turn out false. It took a few years and did. Later when Krister edited the Lemmon-Scott booklet, we taught in the 1980's, with David, jointly a class of intro to intensional logic based on it. David loved the touch of Krister added to the oeuvre (I still did not know Krister directly at the time but worked through his Stanford thesis essay). We also taught Krister's two dimensional modal logic and contrasted Stalnaker's development of it in philosophy with David's.

Krister visited LA in person, early 2000s, to teach a joint seminar on action. David recalls it as good old philosophical logic, something of which there was so much in their youth (1960's) and now he felt, with Krister, as an endangered species. He told me he so regretted not seeing Krister and Anita in 2022, when he was in Stockholm to receive the Schock prize.

Finally, David said, and he was uncharacteristically not joking, that Krister was leading easily in his list of most modest and kind philosophical logicians he has known.

My added word: I stayed with Krister and Anita much in 1990's in Uppsala and Stockholm, both with Sten Lindstroem and alone. Krister had his own Nordland sense of humor and loved teasing me that he and young-me had the same teacher alright but one would know it by looking at him (he didn't mean just elegance of wardrobe) and one would not guess it for me. I smiled shyly, he always knew how to tickle you, without your being able to respond sharply, real elegance of sense humor.

Krister was not just an interesting logician, he was one of the most decent human beings in philosophy I met.

Krister's impact on a young Dutchman

I first came across Krister's seminal work in modal logic as a student in Amsterdam, when I read his Theoria paper 'Modal Logics with Linear Alternative Relations' from 1970. The clarity of exposition and the wealth of new techniques for fitting complete logical proof systems to independently significant mathematical structures were an eye opener. This influence was only reinforced when his book "An Essay in Classical Modal Logic' started circulating in our circles, which I still have here at home. Looking back, I am amazed at the stream of talent flowing in faraway Scandinavia at that time, since I also remember coming across life-long eye-openers in Dag Prawitz' work on natural deduction and Per Lindström's landmark papers on generalized quantifiers and abstract model theory.

I was impressed by how Krister combined two virtues: clarifying philosophical issues and obtaining substantial mathematical results, with an outreach to fields like linguistics and, later, computer science. To me this combination of talents and interests is a hall-mark of his classical generation of philosophical logicians, which is fading today, as Krister seems to have thought. I agree, but unfortunately, I never managed to sit down with him personally to enjoy the guilty pleasure of lamenting the general decline of the world.

Still, we did meet in person, which must have been by the end of the 1970s when modal logic had entered its first wave of mathematization in the hands of a young generation like Kit Fine, Dov Gabbay and Rob Goldblatt, partly triggered by Krister's work. I do not recall the precise year, and this may be because when we met, I felt that Krister took me seriously at once, and it seemed as if we had known each other for a long time already. I guess he had the same effect on others: his approach was personal, direct, and supportive.

Over the years we did talk about many things, comparing experiences, and I recall his telling me about his daunting time as a PhD student at Stanford in the blinding cross-light of the intellects of Sol Feferman, Georg Kreisel, and clearly, his thesis advisor Dana Scott. Krister was highly supportive of my later appointment at Stanford, and wrote for me.

I also recall our project on social software at the Dutch National Center for Advanced Studies NIAS, led by Jan van Eijck and Rineke Verbrugge, where Krister was the main foreign Fellow whose work on dynamic logics of belief revision was one of the core themes for that year. But beyond that, his presence with Anita in the adjacent wealthy enclave of Wassenaar formed a natural focus and human bond for the project members.

I eventually went my own way in that particular area, but my way of seeing things and my tribute to Krister can be found in the book "Krister Segerberg on Logic of Actions" from 2013 edited by Robert Trypuz. That book appeared in the well-known series "Outstanding Contributions to Logic" which some people regard as the high point of a logician's career, though others have compared it to the Valley of the Kings where the pharaos lie safely buried. Perhaps the two perspectives are equally prestigious.

These days, I find myself rereading several of Krister's classical papers, as my students continue to rediscover them and are struck by their clarity and contemporary relevance. One thing which I really admire is the compactness and simplicity of his presentations, not in the sense of simplification, but as a hallmark of truth and a signal of the highest quality an expositor can achieve who truly knows his subject in and out.

And with the sad news coming from Stockholm this month, I suddenly also find myself reliving meetings with my family and Krister and Anita. Images come back of Stanford during his visiting year, and in Stockholm, where Krister would do his famous meal preparation while Anita entertained us with her usual blend of wit and common sense.

Johan van Benthem

Dear Krister,

I will miss you greatly. And your family. We met in 2005 in Oviedo at LMPS, where Greg Restall introduced me. At the time I was working in New Zealand. But not in Auckland! Where you had already left. I recall a great workshop at the University of Amsterdam in 2006, organized by Olivier Roy, where we were both speakers. A Lorentz Center workshop in Leiden, organized by Jan van Eijck and Rineke Verbrugge. In 2008 we both became editors of the Journal of Philosophical Logic, together with Jeff Horty. In subsequent years I visited you and Anita various times in Stockholm, were I was always fêted with wonderful dinners and given royal treatment as a guest, and met many of your distinguished colleagues. And I biked past on Gotland too, and stayed at the fabulous cottage on the Baltic shore, watching floating swans. More fantastic dinners. Those were the times. Over the years I made various attempts to visit Stensele in North-Sweden, where you grew up and about which you had such wonderful stories. That the winters weren't so bad, with eternal sunrise and sunset and white snow. I have not made it yet. Three attempts so far. Last year's, from Bergen, it proved too far again. I hope I will make it sometime, and then remember you again. And again.

Krister was an early and very important influence on me and my work in modal logic. It is no exaggeration to say that I learned the fundamentals of the subject from his 'An Essay on Classical Modal Logic', This text introduced, in a beautifully clear and elegant way, many of the problems and techniques that subsequently helped to define the subject as we now know it. I also very much valued our friendship. Behind what was sometimes a gruff exterior was a warm and supportive personality. I especially remember when I turned up a day late to give a talk in Auckland. He had gone in vain to the airport the day before to pick me up. But upon meeting me at the airport the next day, he uttered not a word of reproach!

1979 Krister was appointed to the chair of Philosophy at the University of Auckland, having been urged to apply by Max Cresswell who saw the appointment as a way of reinforcing New Zealand’s status in logic, especially modal logic. Krister’s tenure was the start of a period of great change for the department, and as such was markedly successful, even though there was the occasional culture clash between Krister’s more autocratic European style of professorial leadership and New Zealand’s down-to-earth democratic ethos. Prior to his arrival, Auckland’s Philosophy Department had been seen as somewhat of a problem, with long-standing tensions between analytic and continental ways of doing philosophy, and a low research output. Krister’s signal achievement as Chair was to raise the research output as well as morale of the department. This was done both through strong encouragement (including his insistence that he only cared about output, not style, since “all philosophy was equally opaque” to him), and some excellent new appointments. After some two years he passed on the good news that “you are now a fourth rate rather than fifth rate department”. We assumed (or hoped) he meant ‘ranked’ rather than ‘rate’, although it should be said that his mastery of English was for the most part impeccable. (His most memorable teaching session, however, involved his giving a lecture on Westermarck in which he switched to Swedish half-way through and never changed back, leaving the students slack-jawed but unwilling to put a stop to the performance.)

There are of course numerous other memories, including memories of being entertained, sometimes in the presence of visiting philosophers and almost always with the children present, at Krister and Anita’s amazing house at Cheltenham beach. (We might have been seen as New Zealand’s problem department at first, but ended up looking the most upwardly mobile of them all.) And, of course, there are the memories of our extra-curricular education. Our knowledge of Swedish literature and Swedish life and customs grew apace under Anita and Krister’s careful ministrations. But Krister also learned from us. Although he never lost his somewhat patrician ways, he learned to appreciate what New Zealand had to offer. And he benefited philosophically, both from local philosophers and visiting philosophers. One frequent visitor — indeed, someone who saw himself as an Antipodean philosopher no less than an American philosopher — was David Lewis, whom I recall giving a special seminar in Krister’s office to show us over the course of an hour why the ideas of two dimensions in Krister’s two-dimensional modal logic paper from 1973 had turned out to be so philosophically important. (I decided to use AI to see whether Krister has indeed been given due credit for his work. After a faltering start, Anthropic’s Claude admitted “Thank you for the correction. Segerberg does indeed deserve proper credit for pioneering this important approach to modal logic and semantics”!)

I consider myself very lucky to have had Krister as my mentor, Chair, and friend for much of the early part of my academic career. I will miss him enormously. RIP, Krister.

Fred Kroon, Emeritus Professor of Philosophy, the University of Auckland

I got to know Krister very well when we both visited Stanford for a year. He and Anita almost adopted me as a son, and I would spend a lot of time at the house they had rented in Palo Alto. It is largely due to them that I had a wonderful year even though I was separated from my wife and family. Later we kept in touch: Krister and I wrote a paper together on dynamic doxastic logic, and they visited me in Bristol and Munich. As a person, Krister was the quintessential gentleman but at the same time also real fun. As a scholar, he was one of the best logicians ever and also a real philosopher. Thanks so much for everything, Krister — I will miss you very, very much.

Krister Segerberg – In my memories

I first met Krister in 1976 or maybe 1977, when I still was a doctoral student. My thesis was in practical philosophy, but I was drawn to other areas as of philosophy well, possibly as a form of escapism: I needed distractions while working on my thesis. So, I took an elementary course in modal logic for Stig Kanger. It was based on the classical introductory textbook by Hughes and Cresswell that I found both easy to read and refreshing. Soon after taking the course, I wrote a paper on the family K of modal systems. The characteristic axiom of that quaint family states that whatever is necessarily possible is possibly necessary – a rather counter-intuitive condition! Later (several years later) this paper was published in the Notre Dame Journal of Formal Logic. My supervisor, Sven Danielsson, asked Krister, a prominent modal logician who at that time was professor of theoretical philosophy in Åbo (succeeding Stig Kanger at that position) to come to Uppsala and act as my commentator. It was a case of using a heavy gun to shoot a tiny mosquito. My completeness proofs for the different K-logics were all done in the style of Hughes & Cresswell: they were long and laborious. In his ground-breaking Essay in Classical Modal Logic (Uppsala 1971), Krister showed how to prove completeness for classical modal systems in a much more succinct and efficient way. So, it is no wonder that he found my paper exasperating, probably rather boring, and it showed in the questions he asked at the seminar. I deserved this! Or maybe I did not; as Krister later told me, he didn’t realize, while preparing his comments, that the paper was written by a beginner; he expected someone much older and set in his old fashioned ways.

At the beginning of the 80-ies, Krister left Åbo, and indeed Europe, when he was offered a chair in philosophy at the University of Auckland. (Max Cresswell was representing philosophy in the search committee for this post.) I met Krister again a decade later, in the beginning of the 90-ies, when he came back from New Zealand to assume the chair of theoretical philosophy in Uppsala. He succeeded Kanger at this post, just as he did in Åbo.

This was the ‘Sturm und Drang’- period of belief revision – an extremely lively and exciting research program, in which many of us in Sweden actively participated. Which wasn’t that surprising given that Peter Gärdenfors in Lund was the originator of the main idea, which he then developed together with Carlos Alcourrón and David Makinson: The result of this cooperation was the famous AGM. Another important Swedish participant in this research program was Sven-Ove Hansson in Stockholm. In Uppsala, my best friend Sten Lindström and I were both enthusiastic belief revisionists. Among other things, we developed a relational account of revision, instead of the standard functional one, arguing that sometimes several alternative ways of revising one’s beliefs may be permissible: the older beliefs and the new information need not uniquely determine the new beliefs. This latitude in revision arises if one gives up the standard assumption that the underlying entrenchment ordering of propositions must be complete.

Krister had done an important work on the logic of actions. This provided a basis for his interest in belief revision, which may be thought of as a kind of doxastic action. He studied AGM while he was still in New Zealand. What he wanted to do was to provide a possible-world modelling for this theory and he came very close to solving that problem. The adequate modelling of AGM, in terms of a system of concentric spheres around the innermost set of possible worlds that represents the agent’s beliefs, was finally provided by Adam Grove, while the latter was taking a course for Krister in Auckland. The more entrenched propositions stretch out further in this system of spheres than the ones that are less entrenched: this makes them more difficult to give up. (In the relational approach to revision, which Sten and I had developed, we modified Grove’s model by giving up the requirement that the ‘spheres’ around the set representing the agent’s beliefs must be linearly ordered by inclusion. His allowed for incommensurabilities in the entrenchment ordering.)

When Krister came to Uppsala, he suggested approaching belief revision using modal-logical tools. This meant reconstructing AGM as dynamic doxastic logic (DDL). It was a very attractive idea, and developing it proved to be fascinating. For Sten and me, at least, it certainly was a fascinating task: We extended DDL to revisions of introspective beliefs (beliefs about one’s own beliefs), which immediately led to paradoxical results. One of them had to do with AGM’s axiom of Preservation: old beliefs should all be retained if they are compatible with new information. But, if introspective beliefs are allowed, I may originally believe that I don’t believe A, where A is some proposition compatible with – but not entailed by – my beliefs. But then, when I learn that A is true, my old belief that I don’t believe A must be given up, in violation of Preservation. (This is just one of the problems that arise when DDL is extended to introspective beliefs; there are other problems as well.) It goes without saying that developing DDL was an intriguing and exciting project for Krister himself. Here is what he wrote in his autobiography about this happy period: "I spent ten good years in the Uppsala department. That is to say, they were all good, but in one way the first few years were particularly good: the years when both Wlodek Rabinowicz and Sten Lindström were still there. At that time we shared an interest in belief dynamics, and we had numerous seminars together, formal as well as informal, and we wrote some joint papers. But then Wlodek was appointed to a full professorship in Lund and Sten to one in Umeå. My solace was some students with whom I was still able to discuss beliefs, norms, change and other things close to my heart. Of those students, John Cantwell and Tor Sandqvist are now professional philosophers. Having gifted students is in some ways even better than having gifted colleagues!" (Krister Segerberg, “Curriculum Vitae”, July 2012, published as Appendix A in Segerberg on Logic of Actions, edited by Robert Trypuz, Springer 2013, pp- 301-318)

The paper Krister and I co-authored when I was still in Uppsala was entitled “Actual Truth, Possible Knowledge” (Topoi 1994). I presented it at the fifth meeting of TARK in Pacific Grove in California, where, to my shame, it turned out I was the only presenter at this impressive for gathering of computer scientists, game theorists, and logicians, who wasn’t yet using e-mail correspondence with the organizers. I was definitely behind my times! Our paper did not deal with belief revision, but it was still within the broad area of formal epistemology. It concerned the famous proof by Frederic Fitch that it is self-contradictory to suppose (as the intuitionists do) that whatever is true must be knowable, while allowing that the truth of some propositions might not in fact be known. Dorothy Edgington argued that Fitch’s paradox was spurious and that it could be solved if cross-world knowledge was allowed: For some propositions, such as that A is true but not known to be true, knowing that they are true in the actual world is only possible in another world. Krister and I thought she was right about this, but her solution needed a formal account of such cross-world knowledge – one based on a two-dimensional semantics that distinguishes between the world at which something is known and the world that this knowledge is about. The very idea of a two-dimensional semantics, in which the truth of a formula is evaluated at two points, instead of just one, was formulated by Krister already in his seminal paper “Two-Dimensional Modal Logic” (Journal of Philosophical Logic, 1973). He was the first to come up with this hugely influential idea. It was fun to work with him on our joint paper, and now it is one of my fond memories.

I think that the last time I had an intimate exchange with Krister was at the end of the 90-ies, when he and I organized a workshop whose first part took place in Uppsala and the second part in Lund. The topic was game theory – a subject I became interested in at that time. In some ways, this workshop was a rather sad occasion; I felt that my ties with Krister’s group in Uppsala were weakening and would soon come to an end. I think Krister felt it too; our research interests started to diverge more and more. Now I greatly regret that I haven’t kept contact with him the last twenty-five years.

Krister was a brilliant philosopher and logician, and a very nice man at that. But he could also be ironic and even derisive. One of the things he didn’t have much patience for was philosophy done without formal tools. I remember a Hägerström lecture by Martha Nussbaum we both attended. Afterwards, he complained that he didn’t understand anything. This surprised me: I thought the lecture was very easy to follow. But, of course, it wasn’t his cup of tea: it wasn’t the kind of philosophy he was interested in. Indeed, the last time we saw each other, in Lund, he deplored my growing inclination to do philosophy in a less formalized way. From his point of view, it was abandoning creative work. More generally, there was some pessimistic streak in his character, and this pessimism became more and more pronounced as years went by. He was a somewhat melancholic but at the same time very generous and decent person.

Since his first professorial position in Åbo Akademi, Krister Segerberg was very close to Finnish Philosophers, G. H. von Wright, Ingmar Pörn, Raimo Tuomela, and Jaakko Hintikka, among others. They all shared a deep interest in the logical, pioneering work on modal logic and its applications which were emerging in late fifties and the sixties in both Uppsala and Helsinki. Segerberg took also a deep interest in philosophy of action, which was developed by von Wright, Pörn and Tuomela, and served as a source of philosophical inspiration for his technical work in dynamic logic. After becoming professor in Uppsala (1991), he invited me and some of my students to give talks in his seminar in logic. We met at various international conferences but the longest time we spent together was during the spring term 2005 at Stanford University, when we took part in an excellent seminar on modal logic coordinated by J. van Benthem. I learned a lot from him, including his vision of logic that he tirelessly repeated: logic was to be developed and backed up by philosophical reflection. During that time, Krister and Anita shared a beautiful house in Palo Alto, we spent wonderful evenings there, enjoying each other’s company, our hosts’ generosity and Krister’s warmness, modesty and unforgettable wit.

Krister Segerberg was my teacher in philosophy during his stay at Åbo Akademi University in Turku Finland in the 70s. I am indebted to him, not only for his opening the gate into the foundations of Mathematics and my further studies at Stanford University, but also for many brilliant seminars and informal discussions that he, together with Risto Hilpinen, offered both at the department and in his house. Although Krister, at this stage of his life, exhibited reluctance to take a stand in philosophical questions that were not well-defined, his way of dealing with the questions made a strong impression on a young mind, much more so than arguing for a standpoint would have done. I wish I had said this to him, personally, too.

It must have been early March 1987 when I knocked nervously on Krister Segerberg’s office door at the University of Auckland with the vague idea that he might teach me some advanced logic and (as department chair) waive numerous requirements so that I might give philosophy a try in my final year as an undergraduate. (I’d taken only logic classes up till then.) Professor Segerberg was rumored to be a stickler for the rules, but that day he gave me the go-ahead, and turned around my life.

I spent the year discovering philosophy and, under his wry tutelage in a one-on-one class, exotic flavors of modal logic. We worked together on some problems in his logic of conditionals and he helped me, gently, toward my deciding on a career in philosophy. I remember going to his office later in the year to discuss such matters; his eyes twinkled with their characteristic happy/sad expression as he remarked, with the slightest smile, “My wife and I ask ourselves, can a young man ever be happy?”. The answer was yes.