Deborah Kant

This chapter examines the philosophical significance of the forcing method through the lens of contemporary set-theoretic practice. Building on the practice-based approach pioneered by Penelope Maddy, it situates forcing within the broader universe-multiverse debate and critically compares the universe view defended by Maddy with Joel D. Hamkins' multiverse pluralism. Although both accounts appeal to mathematical practice, they draw divergent conclusions, raising a methodological puzzle about how practice is to be interpreted. To address this puzzle, the chapter introduces a scope guideline for practice-based philosophy of set theory, according to which philosophical analyses must respect and explain the full diversity of set-theoretic practices rather than privileging a narrow subset. The guideline is applied in a detailed case study of forcing, drawing on qualitative interview data from professional set theorists representing both absolutist and pluralist orientations. The findings show that forcing is widely valued across the community but integrated into distinct research aims. The chapter argues that forcing is largely philosophically neutral, though not uniformly so, and that its methodological versatility reflects the coexistence of multiple epistemic ideals within set theory. More generally, the analysis highlights how practice-based philosophy must accommodate internal diversity if it is to yield credible philosophical conclusions about mathematics.

Frege’s full comprehension scheme, formulated in "Grundgesetze der Arithmetik", was central to his logicist project. In its unrestricted form, however, it led directly to Russell’s paradox and was therefore rejected as a foundation for mathematics. This chapter revisits Frege’s comprehension principle from the perspective of modern set theory. Drawing on historical analysis and contemporary mathematical practice, I argue that although full comprehension cannot be sustained as a formal axiom, an informal version continues to guide set-theoretic reasoning. By examining the blurred boundary between sets and proper classes in current mathematical work, I suggest that Frege’s original idea survives as an informal working principle, applied competently by practitioners without generating contradictions.

Penelope Maddy’s Second Philosophy is one of the most well-known approaches in recent philosophy of mathematics. She applies her second-philosophical method to analyze mathematical methodology by reconstructing historical cases in a setting of means-ends relationsMeans-ends relation. However, outside of Maddy’s own work, this kind of methodological analysis has not yet been extensively used and analyzed. In the present work, we will make a first step in this direction. We develop a general framework that allows us to clarify the procedure and aims of the Second Philosopher’sSecond philosopher investigation into set-theoretic methodologySet-theoretic methodology; provides a platform to analyze the Second Philosopher’sSecond philosopher methods themselves; and can be applied to further questions in the philosophy of set theory.

The International Humanities Council has established a new international research network Diversity of Mathematical Research Cultures & Practices (DMRCP) at the Universität Hamburg. In a tripartite contribution, we outline and discuss the specific philosophical approach that DMRCP seeks to promote for which we use the term ‘empirical philosophy of mathematics’: the contribution is therefore programmatic and methodological, rather than a contribution to a specific philosophical research question. This article forms Part III of the triptych.

Many believe mathematical truth is indisputable. However, the set-theoretic independence phenomenon challenges this idea. Certain statements about infinite sets, like the continuum hypothesis, are neither true nor false according to the standard axioms. While philosophers have offered various diagnoses of this problem, this book posits that the set-theoretic community is key to solving the issue, proposing a pragmatic approach. It presents the first extensive empirical study, featuring interviews with 28 set theorists from varied backgrounds. It explores the spectrum of disagreement and agreement among practitioners, predicting whether the community will adopt new axioms. Overall, this book demonstrates the potential of empirical research for philosophical purposes, also contributing to debates in social epistemology. It addresses a complex problem, applies a novel sociological method, and reveals much about the inner workings of a scientific community in the foundations of mathematics.

Hamkins’ multiverse view is a prominent position on the nature of set theory. It is posited against the universe view and proposed as a philosophical theory explaining current set-theoretic practice. This paper confronts the multiverse view with the results of an interview study investigating current set-theoretic practice. The study reveals a heterogeneity of set-theoretic research practices. The multiverse view is found to align well with pluralist research practices but not with absolutist practices. The generalisation claim of the multiverse view fails because of this heterogeneity; only a reduced interpretation of the multiverse view might hold generally. Furthermore, Hamkins’ prediction that the community as a whole will not adopt axioms deciding the continuum hypothesis is found probably true, but for different reasons: although part of the community has already accepted axioms beyond ZFC, another part of the community will withdraw from doing so. In conclusion, I argue that Hamkins’ multiverse view is best interpreted as a valuable perspective on pluralist set-theoretic practices.

This paper analyses the hidden use of new axioms in set-theoretic practice with a focus on large cardinal axioms and presents a general overview of set-theoretic practices using large cardinal axioms. The hidden use of a new axiom provides extrinsic reasons in support of this axiom via the idea of verifiable consequences, which is especially relevant for set-theoretic practitioners with an absolutist view. Besides that, the hidden use has pragmatic significance for further important sub-groups of the set-theoretic community---set-theoretic practitioners with a pluralist view and set-theoretic practitioners who aim for ZFC-proofs. By describing this, the paper gives a more complete picture of new axioms in set-theoretic practice. These observations, for instance, show that set-theoretic practitioners interested in ZFC-proofs use tools that go beyond ZFC. The analysis is based on empirical data that was collected in an extensive interview study with set-theoretic practitioners.

The status of independent statements is the main problem in the philosophy of set theory. We address this problem by presenting the perspective of a practising set theorist. We thus give an authentic insight in the current state of thinking in set-theoretic practice, which is to a large extent determined by independence results. During several meetings, the second author asked the first author about the development of forcing, the use of new axioms and set-theoretic intuition on independence. Parts of these conversations are directly presented in this article. They are supplemented by important mathematical results as well as discussion sections. Finally, we present three hypotheses about set-theoretic practice: First, that most set theorists were surprised by the introduction of the forcing method. Second, that most set theorists think that forcing is a natural part of contemporary set theory. Third, that most set theorists prefer an answer to a problem with the help of a new axiom of lowest possible consistency strength, and that for most set theorists, a difference in consistency strength weighs much more than the difference between Forcing Axiom and Large Cardinal Axiom.

This work gives a new argument for ‘Empirical Philosophy of Mathematical Practice’. It analyses different modalities on how empirical information can influence philosophical endeavours. We evoke the classical dichotomy between “armchair” philosophy and empirical/experimental philosophy, and claim that the latter should in turn be subdivided in three distinct styles: Apostate speculator, Informed analyst, and Freeway explorer. This is a shift of focus from the source of the information towards its use by philosophers. We present several examples from philosophy of mind/science and ethics on one side and a case study from philosophy of mathematics on the other. We argue that empirically informed philosophy of mathematics is different from the rest in a way that encourages a Freeway explorer approach, because intuitions about mathematical objects are often unavailable for non-mathematicians (since they are sometimes hard to grasp even for mathematicians). This consideration is supported by a case study in set theory.

Penelope Maddy’s Second Philosophy is one of the most well-known ap- proaches in recent philosophy of mathematics. She applies her second-philosophical method to analyze mathematical methodology by reconstructing historical cases in a setting of means-ends relations. However, outside of Maddy’s own work, this kind of methodological analysis has not yet been extensively used and analyzed. In the present work, we will make a first step in this direction. We develop a general framework that allows us to clarify the procedure and aims of the Second Philosopher’s investigation into set-theoretic methodology; pro- vides a platform to analyze the Second Philosopher’s methods themselves; and can be applied to further questions in the philosophy of set theory.

There are different narratives on mathematics as part of our world, some of which are more appropriate than others. Such narratives might be of the form ‘Mathematics is useful’, ‘Mathematics is beautiful’, or ‘Mathematicians aim at theorem-credit’. These narratives play a crucial role in mathematics education and in society as they are influencing people’s willingness to engage with the subject or the way they interpret mathematical results in relation to real-world questions; the latter yielding important normative considerations. Our strategy is to frame current narratives of mathematics from a virtue-theoretic perspective. We identify the practice of mathematizing, put forward by Freudenthal’s ‘Realistic mathematics education’, as virtuous and use it to evaluate different narratives. We show that this can help to render the narratives more adequately, and to provide implications for societal organization.

This edited work presents contemporary mathematical practice in the foundational mathematical theories, in particular set theory and the univalent foundations. It shares the work of significant scholars across the disciplines of mathematics, philosophy and computer science. Readers will discover systematic thought on criteria for a suitable foundation in mathematics and philosophical reflections around the mathematical perspectives. The first two sections focus on the two most prominent candidate theories for a foundation of mathematics. Readers may trace current research in set theory, which has widely been assumed to serve as a framework for foundational issues, as well as new material elaborating on the univalent foundations, considering an approach based on homotopy type theory (HoTT). The further sections then build on this and are centred on philosophical questions connected to the foundations of mathematics. Here, the authors contribute to discussions on foundational criteria with more general thoughts on the foundations of mathematics which are not connected to particular theories. This book shares the work of some of the most important scholars in the fields of set theory (S. Friedman), non-classical logic (G. Priest) and the philosophy of mathematics (P. Maddy). The reader will become aware of the advantages of each theory and objections to it as a foundation, following the latest and best work across the disciplines and it is therefore a valuable read for anyone working on the foundations of mathematics or in the philosophy of mathematics.