# Pluralism in the foundations of mathematics, ASL invited address, joint APA/ASL meeting, New York, January 2024

This will be an invited ASL address at the joint meeting of the ASL with the APA Eastern Division conference, held in New York 15-18 January 2024. My talk will be 16 January 2024 11:00 am.

Abstract. I shall give an account of the debate on set-theoretic pluralism and pluralism generally in the foundations of mathematics, including arithmetic. Is there ultimately just
one mathematical universe, the final background context, in which every mathematical
question has an absolute, determinate answer? Or do we have rather a multiverse of
mathematical foundations? Some mathematicians and philosophers favor a hybrid notion, with pluralism at the higher realms of set theory, but absoluteness for arithmetic.
What grounds are there for these various positions? How are we to adjudicate between
them? What ultimately is the purpose of a foundation of mathematics?

See the related paper on the mathematical thought experiment: How the continuum hypothesis could have been a fundamental axiom.

# Embeddability amongst the countable models of set theory, plenary talk for ASL / Joint Math Meetings in Baltimore, January 2014

A one-hour plenary talk for the ASL at the Joint Math Meetings, January 15-18, 2014 in Baltimore, MD.

Abstract. A surprisingly vigorous embeddability phenomenon has recently been uncovered amongst the countable models of set theory.  In particular, embeddability is linear:  for any two countable models of set theory, one of them is isomorphic to a submodel of the other.  In particular, every countable model of set theory, including every well-founded model, is isomorphic to a submodel of its own constructible universe, so that there is an embedding $j:M\to L^M$ for which $x\in y\iff j(x)\in j(y)$. The proof uses universal digraph combinatorics, including an acyclic version of the countable random digraph, which I call the countable random $\mathbb{Q}$-graded digraph, and higher analogues arising as uncountable Fraïssé limits, leading to the hypnagogic digraph, a set-homogeneous, class-universal, surreal-numbers-graded acyclic class digraph, closely connected with the surreal numbers.