Naive Set Theory Research Articles

On the structural properties of paradoxes: the distinction between formal language and natural language that comes with the use of the liar paradox

This study delves into the intriguing realm of paradoxes that have long fascinated philosophers and logicians throughout history. It begins by discussing the nature and purpose of paradoxes, ranging from their role in entertainment to their capacity to reveal flaws within logical systems. This work emphasizes the challenge paradoxes pose to the completeness of systems and the subsequent development of axiomatic systems that aim to eliminate paradoxes. Rather than providing definitive solutions to paradoxes, the primary aim of this study is to defend the idea that systems containing paradoxes can coexist with completeness. The focus is on categorizing paradoxes, with special attention given to the group known as liar paradoxes, including Russell’s famous variation. The text demonstrates how these paradoxes are intrinsically linked to the principle of non-contradiction and argues that the truth and contradiction of propositions are both the cause and consequence of these paradoxes, presenting a dilemma within the system. The work introduces the concept of Aristotelian Sets (A-Sets) and Empty Sets as potential solutions to these paradoxes. It explores the idea that these sets, when carefully defined, can provide a meaningful representation of individual substances and predicates without violating the principle of non-contradiction. By proposing the inclusion of non-existents within a naive set theory and introducing A-Sets, this work seeks to contribute to the ongoing discourse surrounding paradoxes and their resolution. Ultimately, this study offers a fresh perspective on the handling of paradoxes, emphasizing the importance of reevaluating the foundations of formal and natural languages in the pursuit of a more comprehensive understanding of logic and philosophy.

The import of hypodoxes for the Liar and Russell’s paradoxes

Is the set of all self-membered sets, S, a member of itself? In naive set theory, this is Russell’s hypodox. By the Laws of Excluded Middle and Non-contradiction, S is a member of itself xor it is not, but no principle of classical logic or naive set theory determines which. (Herein, ‘xor’ extends English with a specifically exclusive disjunction.) As a hypodox, the Truth-teller is a sentence that says of itself simply that it is true; by the above mentioned principles, the Truth-teller is true xor not true, but no principle of classical logic or naive truth determines which. Many concepts that have paradoxes have related hypodoxes, although the focus here is on these two. I argue that the Truth-teller does not rely on a principle of truth, in particular it does not rely on the T-schema. I also argue that Russell’s hypodox does not rely on Comprehension. Yet I relate Russell’s paradox and hypodox based on involution functions. The Liar and the Truth-teller are also related by means of involutions. Relations based on these involution functions can be represented in duality squares. Although hypodoxes are not paradoxes, these involution functions and duality relations warrant some commonality in a correct solution to a paradox and its dual hypodox. Using this and some other premises, I provide novel arguments supporting non-classical solutions for the Liar paradox and the significance of Russell’s hypodox for solutions to Russell’s paradox.

The third AI summer: AAAI Robert S. Engelmore Memorial Lecture

The third AI summer: AAAI Robert S. Engelmore Memorial Lecture

LF+ in Coq for "fast and loose" reasoning

We develop the metatheory and the implementation of LF+, and discuss several applications. LF+ capitalizes on research work, carried out by the authors over more than a decade, on Logical Frameworks. It builds on various conservative extensions of LF, which feature lock-type constructors, and on the new perspectives offered by its novel shallow implementation in Coq. The L^P(N:sigma)[.] constructor, and its binding variant L^P(?x:sigma)[.], capture monadically the concept of inhabitability up-to. They were originally introduced for factoring-out, postponing, or delegating to external tools the verification of time-consuming judgments, which are morally proof-irrelevant, thus allowing for integrating different sources of epistemic evidence in a unique Logical Framework. Experimenting with locks has shown that they are also a very flexible tool for expressing in Type Theory several diverse cognitive attitudes and mental strategies used in ordinary reasoning. These range from the emerging paradigm of fast and loose reasoning, which trades off efficiency for correctness, as in naive Set Theory, or in computer architecture and distributed systems, when branch prediction and optimistic concurrency control are implemented. Lock-types naturally express also Typical Ambiguity provisos, squash' types, and many forms of reasoning-up-to.

Fractals for minimalists

The aim of this note is to present an elementary way to fractals which completely avoids advanced analysis and uses purely naive set theory. The approach relies on fixed point methods, where the role of the Banach contraction principle is replaced by a slightly improved version of the Knaster–Tarski fixed point theorem.

What did Frege take Russell to have proved?

In 1902 there arrived in Jena a letter from Russell laying out a proof that shattered Frege’s confidence in logicism, which is widely taken to be the doctrine according to which every truth of arithmetic is re-expressible without relevant loss as a provable truth about a purely logical object. Frege was persuaded that Russell had exposed a pathology in logicism, which faced him with the task of examining its symptoms, diagnosing its cause, assessing its seriousness, arriving at a treatment option, and making an estimate of future prospects. The symptom was the contradiction that had crept into naive set theory in the form of the set that provably is and is not its own member. The diagnosis was that it is caused by Basic Law V of the Grundgesetze (Frege, In: Grundgesetze der Arithmetik: Begrifsschriftlich abgeleitet, Jena: Herman Pohle, 1893/1903. Translated into English as Basic Laws of Arithmetic, also edited by Philip A. Ebert and Marcus Rossberg, with Crispin Wright, Oxford: Oxford University Press, 2013). Triage answers the question, “How bad is it?” The answer was that the contradiction irreparably destroys the logicist project. The treatment option was nil. The disease was untreatable. In due course, the prognosis turned out to be that a scaled-down Fregean logic could have an honourable life as a theory inference for various domains of mathematical discourse, but not for domains containing the logical objects required for logicism. Since there aren’t such objects, there aren’t such domains. On the face of it, Frege’s logicist collapse is astonishing. Why wouldn’t he have repaired the fault in Law V and gone back to the business of bringing logicism to an assured realization? In the course of our reflections, we will have nice occasion to consider the good it might have done Frege to have booked some time with Aristotle had he been able to. By the time we’re finished, we’ll have cause to think that in the end the Russell might well have begged the question against Frege.

Foundations of Transmathematics

Transmathematics has the ambition to be a total mathematics. Many areas of the usual mathematics have been totalised in the transmathematics programme but the totalisations have all been carried out with the usual set theory, ZFC, Zermelo-Fraenkel set theory with the Axiom of Choice. This set theory is adequate but it is, itself, partial. Here we introduce a total set theory as a foundation for transmathematics.
 Surprisingly we adopt naive set theory. It is usually considered that the Russell Paradox demonstrates that naive set theory is incoherent because an apparently well-specified set, the Russell Set, cannot exist. We dissolve this paradox by showing that the specification of the Russell Set admits many unproblematical sets that do not contain themselves and, furthermore, unequivocally requires that the Russell Set does not contain itself because, were it to do so, that one element of the Russell Set would have contradictory membership. Having resolved the Russell Paradox, we go on to make the case that naive set theory is a paraconsistent logic.
 In order to demonstrate the sufficiency of naive set theory, as a basis for transmathematics, we introduce the transordinals. The von Neumann ordinals supply the usual ordinals, the simplest unordered set is identical to transreal nullity, and the Russell Set, excluding nullity, is the greatest ordinal, identical to transreal infinity. The generalisation of the transordinals to the whole of established transmathematics is already known.
 As naive set theory contains all other set theories, it provides a backwardly compatible foundation for the whole of mathematics.

Bi-Modal Naive Set Theory

This paper describes a modal conception of sets, according to which sets are 'potential' with respect to their members. A modal theory is developed, which invokes a naive comprehension axiom schema, modified by adding 'forward looking' and 'backward looking' modal operators. We show that this 'bi-modal' naive set theory can prove modalized interpretations of several ZFC axioms, including the axiom of infinity. We also show that the theory is consistent by providing an S5 Kripke model. The paper concludes with some discussion of the nature of the modalities involved, drawing comparisons with noneism, the view that there are some non-existent objects.

The Hidden Set-Theoretical Paradox of the Tractatus

We are familiar with various set-theoretical paradoxes such as Cantor's paradox, Burali-Forti's paradox, Russell's paradox, Russell-Myhill paradox and Kaplan's paradox. In fact, there is another new possible set-theoretical paradox hiding itself in Wittgenstein’s Tractatus (Wittgenstein 1989). From the Tractatus’s Picture theory of language (hereafter LP) we can strictly infer the two contradictory propositions simultaneously: (a) the world and the language are equinumerous; (b) the world and the language are not equinumerous. I call this antinomy the world-language paradox. Based on a rigorous analysis of the Tractatus, with the help of the technical resources of Cantor’s naive set theory (Cantor in Mathematische Annalen, 46, 481–512, 1895, Mathematische Annalen, 49, 207–246, 1897) and Zermelo-Fraenkel set theory with the axiom of choice (hereafter ZFC) (Jech 2006: 3–15; Kunen 1992: xv–xvi; Bagaria 2008: 619–622), I outline the world-language paradox and assess the unique possible solution plan, i.e., the mathematical plan utilizing ‘infinity’. I conclude that Wittgenstein cannot solve the hidden set-theoretical paradox of the Tractatus successfully unless he gives up his finitism.

METAVALUATIONS

AbstractThis is a general account of metavaluations and their applications, which can be seen as an alternative to standard model-theoretic methodology. They work best for what are called metacomplete logics, which include the contraction-less relevant logics, with possible additions of Conjunctive Syllogism, (A→B) & (B→C) → .A→C, and the irrelevant, A→ .B→A, these including the logic MC of meaning containment which is arguably a good entailment logic. Indeed, metavaluations focus on the formula-inductive properties of theorems of entailment form A→B, splintering into two types, M1- and M2-, according to key properties of negated entailment theorems (see below). Metavaluations have an inductive presentation and thus have some of the advantages that model theory does, but they represent proof rather than truth and thus represent proof-theoretic properties, such as the priming property, if ├ A $\vee$ B then ├ A or ├ B, and the negated-entailment properties, not-├ ∼(A→B) (for M1-logics, with M1-metavaluations) and ├ ∼(A→B) iff ├ A and ├ ∼ B (for M2-logics, with M2-metavaluations). Topics to be covered are their impact on naive set theory and paradox solution, and also Peano arithmetic and Godel’s First and Second Theorems. Interesting to note here is that the familiar M1- and M2-metacomplete logics can be used to solve the set-theoretic paradoxes and, by inference, the Liar Paradox and key semantic paradoxes. For M1-logics, in particular, the final metavaluation that is used to prove the simple consistency is far simpler than its correspondent in the model-theoretic proof in that it consists of a limit point of a single transfinite sequence rather than that of a transfinite sequence of such limit points, as occurs in the model-theoretic approach. Additionally, it can be shown that Peano Arithmetic is simply consistent, using metavaluations that constitute finitary methods. Both of these results use specific metavaluational properties that have no correspondents in standard model theory and thus it would be highly unlikely that such model theory could prove these results in their final forms.

An informational interpretation of weak relevant logic and relevant property theory

This paper extends the theory of situated inference from Mares (Relevant logic: a philosophical interpretation. Cambridge University Press, Cambrdge, 2004) to treat two weak relevant logics, B and DJ. These logics are interesting because they can be used as bases for consistent naive theories, such as naive set theory. The concepts of a situation and of information that are employed by the theory of situated inference are used to justify various aspects of these logics and to give an interpretation of the notion of set that is represented in the naive set theories that are based on them.

Ekman’s Paradox

Prawitz observed that Russell’s paradox in naive set theory yields a derivation of absurdity whose reduction sequence loops. Building on this observation, and based on numerous examples, Tennant claimed that this looping feature, or more generally, the fact that derivations of absurdity do not normalize, is characteristic of the paradoxes. Striking results by Ekman show that looping reduction sequences are already obtained in minimal propositional logic, when certain reduction steps, which are prima facie plausible, are considered in addition to the standard ones. This shows that the notion of reduction is in need of clarification. Referring to the notion of identity of proofs in general proof theory, we argue that reduction steps should not merely remove redundancies, but must respect the identity of proofs. Consequentially, we propose to modify Tennant’s paradoxicality test by basing it on this refined notion of reduction.

Category theory and set theory as theories about complementary types of universals

Instead of the half-century old foundational feud between set theory and category theory, this paper argues that they are theories about two different complementary types of universals. The set-theoretic antinomies forced naive set theory to be reformulated using some iterative notion of a set so that a set would always have higher type or rank than its members. Then the universal u F = {x | F(x)} for a property F(.) could never be self-predicative in the sense of u F ∈ u F . But the mathematical theory of categories, dating from the mid-twentieth century, includes a theory of always-self-predicative universals – which can be seen as forming the “other bookend” to the never-self-predicative universals of set theory. The self-predicative universals of category theory show that the problem in the antinomies was not self-predication per se, but negated self-predication. They also provide a model (in the Platonic Heaven of mathematics) for the self-predicative strand of Plato’s Theory of Forms as well as for the idea of a “concrete universal” in Hegel and similar ideas of paradigmatic exemplars in ordinary thought.

Paths to Triviality

This paper presents a range of new triviality proofs pertaining to naive truth theory formulated in paraconsistent relevant logics. It is shown that excluded middle together with various permutation principles such as A → (B → C)⊩B → (A → C) trivialize naive truth theory. The paper also provides some new triviality proofs which utilize the axioms ((A → B)∧(B → C)) → (A → C) and (A → ¬A) → ¬A, the fusion connective and the Ackermann constant. An overview over various ways to formulate Leibniz’s law in non-classical logics and two new triviality proofs for naive set theory are also provided.

Sets and supersets

It is a commonplace of set theory to say that there is no set of all well-orderings nor a set of all sets. We are implored to accept this due to the threat of paradox and the ensuing descent into unintelligibility. In the absence of promising alternatives, we tend to take up a conservative stance and tow the line: there is no universe (Halmos, in: Naive set theory, 1960). In this paper, I am going to challenge this claim by taking seriously the idea that we can talk about the collection of all the sets and many more collections beyond that. A method of articulating this idea is offered through an indefinitely extending hierarchy of set theories. It is argued that this approach provides a natural extension to ordinary set theory and leaves ordinary mathematical practice untouched.

REMARKS ON NAIVE SET THEORY BASED ONLP

AbstractDialetheism is the metaphysical claim that there are true contradictions. And based on this view, Graham Priest and his collaborators have been suggesting solutions to a number of paradoxes. Those paradoxes include Russell’s paradox in naive set theory. For the purpose of dealing with this paradox, Priest is known to have argued against the presence of classical negation in the underlying logic of naive set theory. The aim of the present paper is to challenge this view by showing that there is a way to handle classical negation.

NAIVE SET THEORY AND NONTRANSITIVE LOGIC

AbstractIn a recent series of papers, I and others have advanced new logical approaches to familiar paradoxes. The key to these approaches is to accept full classical logic, and to accept the principles that cause paradox, while preventing trouble by allowing a certain sort ofnontransitivity. Earlier papers have treated paradoxes of truth and vagueness. The present paper will begin to extend the approach to deal with the familiar paradoxes arising in naive set theory, pointing out some of the promises and pitfalls of such an approach.

The Importance of Developing a Foundation for Naive Category Theory

Recently Feferman (Rev. Symb. Logic 6: 6–15, 2013) has outlined a program for the development of a foundation for naive category theory. While Ernst (ibid. 8: 306–327, 2015) has shown that the resulting axiomatic system is still inconsistent, the purpose of this note is to show that nevertheless some foundation has to be developed before naive category theory can replace axiomatic set theory as a foundational theory for mathematics. It is argued that in naive category theory currently a ‘cookbook recipe’ is used for constructing categories, and it is explicitly shown with a formalized argument that this “foundationless” naive category theory therefore contains a paradox similar to the Russell paradox of naive set theory.

A workshop for high school students on naive set theory

In this article we present the prototype of a workshop on naive set theory designed for high school students in or around the seventh year of primary education. Our concept is based on two events which the author organized in 2006 and 2010 for students of elementary school and high school, respectively. The article also includes a practice report on the two workshop. Events for high school students or students of elementary school in altering configurations and with different aims are a regular component of modern university teaching portfolio. The importance of events that supplement regular classes on all levels of school education increases constantly. In order to support and encourage interested students and to attract prospective university students the universities and the schools organize events such as open days, information sessions, workshops etc. When planning an event of the above type, it is a natural question, which topic should be discussed and which teaching methods should come into play. In the case of a workshop for high school students at the university the topic should give an insight into academic life. In addition, the university teaching and learning concepts should be applied during the workshop in order to give the participants also an insight in their prospective st udent life. In the case of a course at high or elementary school, the topics covered by the usual curriculum should be supplemented and the participants should be challenged according to their personal level of performance−possibly in contrast to regular classes. In order to meet the above objectives, a workshop must include exercise sessions in which the students work on questions individually or in small groups. The level of support during these exercise sessions must vary from intensive discussions to no support at all. Exercises form a substantial part of mathematical studies, in particular homework which has to be done without any assistance from a teacher is indispensable for mathematics education. The choice of a suitable topic, on the one hand challenging and on the other hand not of unreachable difficulty, in combination with the design of a workshop concept with the above features is a non-trivial task for the designated organizer. This paper is based on two different events which the author organized in 2006 and 2010: In Section 4 we report on a workshop for students of elementary school in their third or fourth year of primary education which was part of a program initiated by a regional group of the German Association for Highly Talented Children (DGhK-OWL). The second event, on which we report in Section 4, was a workshop for high school students in their seventh or eighth year of primary education. It was carried out in the context of the Girl's Day 2010 and took place at the University of Paderborn. In Section 3 we present the prototype of a workshop on naive set theory based on the aforementioned events. Before this, we outline our motivation for this topic in the context of student workshops in Section 2 below.

On the crispness of and arithmetic with a bisimulation in a constructive naive set theory

On the crispness of and arithmetic with a bisimulation in a constructive naive set theory