In analogy with the ancient views on potential as opposed to actual infinity, set-theoretic potentialism is the philosophical position holding that the universe of set theory is never fully completed, but rather has a potential character, with greater parts of it becoming known to us as it unfolds. In this talk, I should like to undertake a mathematical analysis of the modal commitments of various specific natural accounts of set-theoretic potentialism. After developing a general model-theoretic framework for potentialism and describing how the corresponding modal validities are revealed by certain types of control statements, which we call buttons, switches, dials and ratchets, I apply this analysis to the case of set-theoretic potentialism, including the modalities of true-in-all-larger-V_β, true-in-all-transitive-sets, true-in-all-Grothendieck-Zermelo-universes, true-in-all-countable-transitive-models and others. Broadly speaking, the height-potentialist systems generally validate exactly S4.3 and the height-and-width-potentialist systems generally validate exactly S4.2. Each potentialist system gives rise to a natural accompanying maximality principle, which occurs when S5 is valid at a world, so that every possibly necessary statement is already true.  For example, a Grothendieck-Zermelo universe V_κ, with κ inaccessible, exhibits the maximality principle with respect to assertions in the language of set theory using parameters from V_κ just in case κ is a Σ₃-reflecting cardinal, and it exhibits the maximality principle with respect to assertions in the potentialist language of set theory with parameters just in case it is fully reflecting V_κ < V.

This is current joint work with Øystein Linnebo, in progress, which builds on some of my prior work with George Leibman and Benedikt Löwe in the modal logic of forcing. Comments and questions can be made on the speaker’s blog.

This will be the dissertation defense of the speaker.  There will be a one-hour presentation, followed by questions posed by the dissertation committee, and afterwards including some questions posed by the general audience. The dissertation committee consists of Joel David Hamkins (supervisor), Gunter Fuchs, Arthur Apter, Roman Kossak and Philipp Rothmaler.

I will show that Diamond Plus holds in inner models of the form L[A], for subsets A of aleph one in the sense of L[A]. Putting this together with the result from last meeting, that Diamond Plus implies the Kurepa Hypothesis, I will show that if the Kurepa Hypothesis fails, then aleph two is an inaccessible cardinal in L. Again, putting this together with another result from the previous seminar meeting, that one can force the failure of Kurepa’s Hypothesis over a model with an inaccessible cardinal, this shows the equiconsistency of the failure of Kurepa’s Hypothesis with an inaccessible cardinal, over ZFC. These results are mainly due to Silver and Solovay.