Jon Michael Dunn

(1941 - 2021)

One of the most dominant approaches to semantics for relevant (and many paraconsistent) logics is the Routley–Meyer semantics involving a ternary relation on points. To some (many?), this ternary relation has seemed like a technical trick devoid of an intuitively appealing philosophical story that connects it up with conditionality in general. In this paper, we respond to this worry by providing three different philosophical accounts of the ternary relation that correspond to three conceptions of conditionality. We close by briefly discussing a general conception of conditionality that may unify the three given conceptions.

I give a kind of intellectual history of the so-called “Belnap-Dunn Four-valued Logic,” examining its evolution: the 4-element De Morgan lattice of Antonio Monteiro, and related work of Bialynicki – Birula and Helena Rasiowa, and John Kalman; Timothy Smiley’s 4-element matrix for Belnap’s Tautological Entailment; Dunn’s interpretation in terms of “aboutness;” Bas van Fraassen’s semantics for Tautological Entailment using “facts;” and Dunn’s interpretation in terms of how a sentence can be assigned both true and false, or neither true nor false, as well as the usual two values simply true, or simply false. Of course I discuss Nuel Belnap’s viewing the four values as elements in a “bi-lattice” and his famous use of this interpretation for “How a Computer Should think.” I also examine relationships to Richard Routley and Valerie Routley’s “star semantics.” Moreover, I discuss extension of the 4-valued semantics to the whole system R (allowing nested relevant implications), focusing especially on Edwin Mares’ work.I then examine later adaptations and extensions of the Four-valued Logic, including work by Yaroslav Shramko, Tatsutoshi Takenaka, Dunn, Heinrich Wansing, and Hitoshi Omori on “trilattices.” I also explain my recent extension to an infinite valued “Opinion Tetrahedron” (extending Audun Jøsang’s Opinion Triangle) which has the four values as its apexes.I end by acknowledging that ideas involving the 4-values date back to classical Indian logic (Sanjay’s “Four Corners”), prior to the sixth century B.C.E.

Classically, an argument A therefore B is ‘valid’ (or A is said to ‘entail’ B) if and only if (iff) each situation (model) is such that either A is false or B is true. This fits well with so-called ‘tableau’ methods for showing that A entails B by working out the mutual inconsistency of A and ~B. But both the classical notion of validity and the corresponding tableau methods allow that A may entail B because of some feature of A alone, irrespective of B, and vice versa. Thus if A is a contradiction, then each situation is such that A is false, and so a fortiori is such that A is false or B is true. And if A is a contradiction, then a tableau construction will show that A is inconsistent, and so a fortiori that A and ~B are inconsistent. Of course, the same points can be made dually when B is a logical truth.

This paper presents an overview of the pioneering contributions of Larisa Maksimova to relevance logics. She is one of the first researchers who set out to methodically study systems of relevance logics, initially, focusing on Ackermann’s $$\varPi '$$ Π ′ of “Rigorous Implication,” and then extending her work to Anderson and Belnap’s systems E of Entailment and R of Relevant Implication, and other related logics. Not only did she develop an algebraic semantics for E, but also we find that a semantic definition of entailment via a ternary accessibility relation appears — the very first time — in an abstract by her.

Saul Kripke’s work on the semantics of modal logics is well known, unlike his work on Anderson and Belnap’s system E of Entailment (a modal relevance logic), which included his proof of the decidability of its implicational fragment E_>, and also a counterexample to the conjecture of Belnap that E_> is the intersection of the implicational fragments of the relevance logic R and the modal logic S4. This led to Storrs McCall’s suggesting that the “mingle” axiom might be added to E_>, and that this system might be used instead of S4_>. We give a counterexample to this conjecture. We also disprove the conjecture Anderson and Belnap attribute to McCall, in which “restricted mingle” is added to E_>. We use a modal extension of the lattice M_0 and follow ideas of Meyer. In the 1970s, Dunn showed that RM (R with the mingle axiom) can be modeled using a binary accessibility relation, like the accessibility relation in Kripke’s semantics for normal modal logics and his semantics for intuitionistic logic. Dunn also gave a variant interpretation for EM (E with the mingle axiom). We explore some other variations of these binary relational semantics.

The challenge of giving a semantics for relevance logic in terms of worlds or situations intrigued several logicians. As a solution, Fine gave a two-sorted semantics. We overview the semantics as well as some further work of Fine in the area of relevance logic. Then we show that beyond supplying technical results such as soundness, completeness and the finite model property (fmp) for many logics, the operational–relational semantics provides footing for an informal interpretation and it naturally leads to an interpretation in terms of relevance.

A 'Kripke-style' semantics is given for combinatory logic using frames with a ternary accessibility relation, much as in the Tourley-Meyer semantics for relevance logic. We prove by algebraic means a completeness theorem for combinatory logic, by proving a representation theorem for 'combinatory posets.' A philosophical interpretation is given of the models, showing that an element of a combinatory poset can be understood simultaneously as a set of states and as a set of actions on states. This double interpretation allows for one such element to be applied to another. Application turns out to be modeled the same way as 'fusion' in relevance logic.We also introduce 'dual combinators' that apply from the right. We then explore relationships to some well-known substructural logics, showing that each can be embedded into the structurally free. non-associative Lambek calculus, with the embedding taking a theorem ω to a statement of the form Γ ⊨ ω, where Γ is some fusion of the combinators needed to justify the structural assumptions of the given substructural logic. This builds on earlier ideas from Belnap and Meyer about a Gentzen system wherein structural rules are replaced with rules for introducing combinators. We develop such a system and prove a cut theorem.

Prominent philosophers have argued that contradictions contain either too much or too little information to be useful. We dispute this with what we call the “Paradox of the Two Firefighters.” Suppose you are awakened in your hotel room by a fire alarm. You open the door. You see three possible ways out: left, right, straight ahead. You see two firefighters. One says there is exactly one safe route and it is to your left. The other says there is exactly one safe route and it is to your right. While the two firemen are giving you contradictory information, they are also both giving you the perhaps useful information that there is a safe way out and it is not straight ahead. We give two analyses. The first uses the “Opinion Tetrahedron,” introduced by Dunn as a generalization of Audun Jøsang’s “Opinion Triangle.” The Opinion Tetrahedron in effect embeds the values of the “Belnap-Dunn 4-valued Logic” into a context of subjective probability generalized to allow for degrees of belief, disbelief, and two kinds of uncertainty—that in which the reasoner has too little information and that in which the reasoner has too much information. Jøsang had only a single value for uncertainty. We also present an alternative solution, again based on subjective probability but of a more standard type. This solution builds upon “linear opinion pooling.” Kiefer had already developed apparatus for assessing risk using expert opinion, and this influences the second solution. Finally, we discuss how these solutions might apply to “Big Data” and the World Wide Web.

The decidability of the logic of pure ticket entailment means that the problem of inhabitation of simple types by combinators over the base { B, B′, I, W } is decidable too. Type-assignment systems are often formulated as natural deduction systems. However, our decision procedure for this logic, which we presented in earlier papers, relies on two sequent calculi and it does not yield directly a combinator for a theorem of ${T_\to}$. Here we describe an algorithm to extract an inhabitant from a sequent calculus proof—without translating the proof into another proof system.

Arnon Avron has written: “Dunn-McCall logic RM is by far the best understood and the most well-behaved in the family of logics developed by the school of Anderson and Belnap.” I agree. There is the famous saying: “Do not let the perfect become the enemy of the good.” I might say: “good enough.” In this spirit, I will examine the logic R-Mingle, exploring how it is only a “semi-relevant logic” but still a paraconsistent logic. I shall discuss the history of RM, and compare RM to Anderson and Belnap’s system R of relevant implication and to classical two-valued logic. There is a “consumer’s guide,” evaluating these logics as “tools,” in the light of my recent work on “Humans as Rational Toolmaking Animals.”

This is a collection of invited papers from the 1975 International Sym posium on Multiple-valued Logic. Also included is an extensive bib liography of works in the field of multiple-valued logic prior to 1975 - this supplements and extends an earlier bibliography of works prior to 1965, by Nicholas Rescher in his book Many-Valued Logic, McGraw-Hill, 1969. There are a number of possible reasons for interest in the present volume. First, the range of various uses covered in this collection of papers may be taken as indicative of a breadth which occurs in the field of multiple-valued logic as a whole - the papers here can do no more than cover a small sample: question-answering systems, analysis of computer hazards, algebraic structures relating to multiple-valued logic, algebra of computer programs, fuzzy sets. Second, a large part of the interest in such uses and applications has occurred in the last twenty, even ten years. It would be too much to expect this to be reflected in Rescher's 1969 book. Third, in the 1970's a series of annual symposia have been held on multiple-valued logic, which have brought much of this into a sharp focus. * The 1971 and 1972 symposia were held at the SUNY at Buffalo, the 1973 symposium at the Uni versity of Toronto, and the 1974 symposium at West Virginia Uni versity. Papers from these symposia are included in the bibliography which may be found in an appendix of this book.

This paper combines two classes of generalized logics, one of which is the class of weakly implicative logics introduced by Cintula and the other of which is the class of gaggle logics introduced by Dunn. For this purpose we introduce implicational tonoid logics. More precisely, we first define implicational tonoid logics in general and examine their relation to weakly implicative logics. We then provide algebraic semantics for implicational tonoid logics. Finally, we consider relational semantics, called Routley–Meyer–style semantics, for finitary those logics.