Structural connections between a forcing class and its modal logic

  • J. D. Hamkins, G. Leibman, and B. Löwe, “Structural connections between a forcing class and its modal logic,” Israel J. Math., vol. 207, iss. 2, pp. 617-651, 2015.  
    @article {HamkinsLeibmanLoewe2015:StructuralConnectionsForcingClassAndItsModalLogic,
    AUTHOR = {Hamkins, Joel David and Leibman, George and Löwe,
    TITLE = {Structural connections between a forcing class and its modal
    JOURNAL = {Israel J. Math.},
    FJOURNAL = {Israel Journal of Mathematics},
    VOLUME = {207},
    YEAR = {2015},
    NUMBER = {2},
    PAGES = {617--651},
    ISSN = {0021-2172},
    MRCLASS = {03E40 (03B45)},
    MRNUMBER = {3359713},
    DOI = {10.1007/s11856-015-1185-5},
    url = {},
    eprint = {1207.5841},
    archivePrefix = {arXiv},
    primaryClass = {math.LO},

The modal logic of forcing arises when one considers a model of set theory in the context of all its forcing extensions, interpreting $\square$ as “in all forcing extensions” and $\Diamond$ as “in some forcing extension”. In this modal language one may easily express sweeping general forcing principles, such as $\Diamond\square\varphi\to\square\Diamond\varphi$, the assertion that every possibly necessary statement is necessarily possible, which is valid for forcing, or $\Diamond\square\varphi\to\varphi$, the assertion that every possibly necessary statement is true, which is the maximality principle, a forcing axiom independent of but equiconsistent with ZFC (see A simple maximality principle).

Every definable forcing class similarly gives rise to the corresponding forcing modalities, for which one considers extensions only by forcing notions in that class. In previous work, we proved that if ZFC is consistent, then the ZFC-provably valid principles of the class of all forcing are precisely the assertions of the modal theory S4.2 (see The modal logic of forcing). In this article, we prove that the provably valid principles of collapse forcing, Cohen forcing and other classes are in each case exactly S4.3; the provably valid principles of c.c.c. forcing, proper forcing, and others are each contained within S4.3 and do not contain S4.2; the provably valid principles of countably closed forcing, CH-preserving forcing and others are each exactly S4.2; and the provably valid principles of $\omega_1$-preserving forcing are contained within S4.tBA. All these results arise from general structural connections we have identified between a forcing class and the modal logic of forcing to which it gives rise, including the connection between various control statements, such as buttons, switches and ratchets, and their corresponding forcing validities. These structural connections therefore support a forcing-only analysis of other diverse forcing classes.

Preprints available at:  ar$\chi$iv | NI12055-SAS | UvA ILLC PP-2012-19 | HBM 446

The set-theoretical multiverse

  • J. D. Hamkins, “The set-theoretic multiverse,” Review of Symbolic Logic, vol. 5, pp. 416-449, 2012.  
    AUTHOR = {Joel David Hamkins},
    TITLE = {The set-theoretic multiverse},
    JOURNAL = {Review of Symbolic Logic},
    YEAR = {2012},
    volume = {5},
    number = {},
    pages = {416--449},
    month = {},
    note = {},
    url = {},
    doi = {10.1017/S1755020311000359},
    abstract = {},
    keywords = {},
    source = {},
    eprint = {1108.4223},
    archivePrefix = {arXiv},
    primaryClass = {math.LO},

The multiverse view in set theory, introduced and argued for in this article, is the view that there are many distinct concepts of set, each instantiated in a corresponding set-theoretic universe. The universe view, in contrast, asserts that there is an absolute background set concept, with a corresponding absolute set-theoretic universe in which every set-theoretic question has a definite answer. The multiverse position, I argue, explains our experience with the enormous diversity of set-theoretic possibilities, a phenomenon that challenges the universe view. In particular, I argue that the continuum hypothesis is settled on the multiverse view by our extensive knowledge about how it behaves in the multiverse, and as a result it can no longer be settled in the manner formerly hoped for.

Multiversive at n-Category Cafe | Multiverse on Mathoverflow

The set-theoretical multiverse: a natural context for set theory, Japan 2009

  • J. D. Hamkins, “The Set-theoretic Multiverse : A Natural Context for Set Theory,” Annals of the Japan Association for Philosophy of Science, vol. 19, pp. 37-55, 2011.  
    author="Joel David Hamkins",
    title="The Set-theoretic Multiverse : A Natural Context for Set Theory",
    journal="Annals of the Japan Association for Philosophy of Science",
    publisher="the Japan Association for Philosophy of Science",

This article is based on a talk I gave at the conference in honor of the retirement of Yuzuru Kakuda in Kobe, Japan, March 7, 2009. I would like to express my gratitude to Kakuda-sensei and the rest of the logic group in Kobe for the opportunities provided to me to participate in logic in Japan. In particular, my time as a JSPS Fellow in the logic group at Kobe University in 1998 was a formative experience. I was part of a vibrant research group in Kobe; I enjoyed Japanese life, learned to speak a little Japanese and made many friends. Mathematically, it was a productive time, and after years away how pleasant it is for me to see that ideas planted at that time, small seedlings then, have grown into tall slender trees.

Set theorists often take their subject as constituting a foundation for the rest of mathematics, in the sense that other abstract mathematical objects can be construed fundamentally as sets. In this way, they regard the set-theoretic universe as the universe of all mathematics. And although many set-theorists affirm the Platonic view that there is just one universe of all sets, nevertheless the most powerful set-theoretic tools developed over the past half century are actually methods of constructing alternative universes. With forcing and other methods, we can now produce diverse models of ZFC set theory having precise, exacting features. The fundamental object of study in set theory has thus become the model of set theory, and the subject consequently begins to exhibit a category-theoretic second-order nature. We have a multiverse of set-theoretic worlds, connected by forcing and large cardinal embeddings like constellations in a dark sky. In this article, I will discuss a few emerging developments illustrating this second-order nature. The work engages pleasantly with various philosophical views on the nature of mathematical existence.



Some second order set theory

  • J. D. Hamkins, “Some second order set theory,” in Logic and its applications, R.~Ramanujam and S.~Sarukkai, Eds., Springer, 2009, vol. 5378, pp. 36-50.  
    AUTHOR = {Hamkins, Joel David},
    TITLE = {Some second order set theory},
    BOOKTITLE = {Logic and its applications},
    SERIES = {Lecture Notes in Comput.~Sci.},
    VOLUME = {5378},
    PAGES = {36--50},
    PUBLISHER = {Springer},
    EDITOR = {R.~Ramanujam and S.~Sarukkai},
    ADDRESS = {},
    YEAR = {2009},
    MRCLASS = {03E35 (03B45 03E40)},
    MRNUMBER = {2540935 (2011a:03053)},
    DOI = {10.1007/978-3-540-92701-3_3},
    URL = {},

This article surveys two recent developments in set theory sharing an essential second-order nature, namely, the modal logic of forcing, oriented upward from the universe of set theory to its forcing extensions; and set-theoretic geology, oriented downward from the universe to the inner models over which it arises by forcing. The research is a mixture of ideas from several parts of logic, including, of course, set theory and forcing, but also modal logic, finite combinatorics and the philosophy of mathematics, for it invites a mathematical engagement with various philosophical views on the nature of mathematical existence.

The modal logic of forcing

  • J. D. Hamkins and B. Löwe, “The modal logic of forcing,” Trans.~AMS, vol. 360, iss. 4, pp. 1793-1817, 2008.  
    AUTHOR = {Hamkins, Joel David and Löwe, Benedikt},
    TITLE = {The modal logic of forcing},
    JOURNAL = {Trans.~AMS},
    FJOURNAL = {Transactions of the American Mathematical Society},
    VOLUME = {360},
    YEAR = {2008},
    NUMBER = {4},
    PAGES = {1793--1817},
    ISSN = {0002-9947},
    MRCLASS = {03E40 (03B45)},
    MRNUMBER = {2366963 (2009h:03068)},
    MRREVIEWER = {Andreas Blass},
    DOI = {10.1090/S0002-9947-07-04297-3},
    URL = {},
    eprint = {math/0509616},
    archivePrefix = {arXiv},
    primaryClass = {math.LO},
    file = F,

What are the most general principles in set theory relating forceability and truth? As with Solovay’s celebrated analysis of provability, both this question and its answer are naturally formulated with modal logic. We aim to do for forceability what Solovay did for provability. A set theoretical assertion $\psi$ is forceable or possible, if $\psi$ holds in some forcing extension, and necessary, if $\psi$ holds in all forcing extensions. In this forcing interpretation of modal logic, we establish that if ZFC is consistent, then the ZFC-provable principles of forcing are exactly those in the modal theory known as S4.2.

Follow-up article:  Structural connections between a forcing class and its modal logic

A simple maximality principle

  • J. D. Hamkins, “A simple maximality principle,” J.~Symbolic Logic, vol. 68, iss. 2, pp. 527-550, 2003.  
    AUTHOR = {Hamkins, Joel David},
    TITLE = {A simple maximality principle},
    JOURNAL = {J.~Symbolic Logic},
    FJOURNAL = {The Journal of Symbolic Logic},
    VOLUME = {68},
    YEAR = {2003},
    NUMBER = {2},
    PAGES = {527--550},
    ISSN = {0022-4812},
    CODEN = {JSYLA6},
    MRCLASS = {03E35 (03E40)},
    MRNUMBER = {1976589 (2005a:03094)},
    MRREVIEWER = {Ralf-Dieter Schindler},
    DOI = {10.2178/jsl/1052669062},
    URL = {},
    month = {},
    eprint = {math/0009240},
    archivePrefix = {arXiv},
    primaryClass = {math.LO},

In this paper, following an idea of Christophe Chalons, I propose a new kind of forcing axiom, the Maximality Principle, which asserts that any sentence$\varphi$ holding in some forcing extension $V^{\mathbb{P}}$ and all subsequent extensions $V^{\mathbb{P}*\mathbb{Q}}$ holds already in $V$. It follows, in fact, that such sentences must also hold in all forcing extensions of $V$. In modal terms, therefore, the Maximality Principle is expressed by the scheme $(\Diamond\Box\varphi)\to\Box\varphi$, and is equivalent to the modal theory S5. In this article, I prove that the Maximality Principle is relatively consistent with ZFC. A boldface version of the Maximality Principle, obtained by allowing real parameters to appear in $\varphi$, is equiconsistent with the scheme asserting that $V_\delta$ is an elementary substructure of $V$ for an inaccessible cardinal $\delta$, which in turn is equiconsistent with the scheme asserting that ORD is Mahlo. The strongest principle along these lines is the Necessary Maximality Principle, which asserts that the boldface MP holds in V and all forcing extensions. From this, it follows that $0^\sharp$ exists, that $x^\sharp$ exists for every set $x$, that projective truth is invariant by forcing, that Woodin cardinals are consistent and much more. Many open questions remain.

The set-theoretic multiverse: a model-theoretic philosophy of set theory, Paris, 2010

A talk at the Philosophy and Model Theory conference held June 2-5, 2010 at the Université Paris Ouest Nanterre.

Set theorists commonly regard set theory as an ontological foundation for the rest of mathematics, in the sense that other abstract mathematical objects can be construed fundamentally as sets, enjoying a real mathematical existence as sets accumulate to form the universe of all sets. The Universe view—perhaps it is the orthodox view among set theorists—takes this universe of sets to be unique, and holds that a principal task of set theory is to discover its fundamental truths. For example, on this view, interesting set-theoretical questions, such as the Continuum Hypothesis, will have definitive final answers in this universe. Proponents of this view point to the increasingly stable body of regularity features flowing from the large cardinal hierarchy as indicating in broad strokes that we are on the right track towards these final answers.

A paradox for the orthodox view, however, is the fact that the most powerful tools in set theory are most naturally understood as methods for constructing alternative set-theoretic universes. With forcing and other methods, we seem to glimpse into alternative mathematical worlds, and are led to consider a model-theoretic, multiverse philosophical position. In this talk, I shall describe and defend the Multiverse view, which takes these other worlds at face value, holding that there are many set-theoretical universes. This is a realist position, granting these universes a full mathematical existence and exploring their interactions. The multiverse view remains Platonist, but it is second-order Platonism, that is, Platonism about universes. I shall argue that set theory is now mature enough to fruitfully adopt and analyze this view. I shall propose a number of multiverse axioms, provide a multiverse consistency proof, and describe some recent results in set theory that illustrate the multiverse perspective, while engaging pleasantly with various philosophical views on the nature of mathematical existence.

Slides  | Article | see related Singapore talk