Models of Arbitrary Sets

Abstract: There is a growing literature on arbitrary objects as other types of mathematical objects, besides particular mathematical objects. In this paper we study arbitrary sets in the context of ZFC set theory. An arbitrary object is one that is determined by a state space across which it varies and by its instantiations. We compare two semantic strategies. First, we analyse the possibility of modeling arbitrary sets using direct powers of a ZFC structure. This yields a natural setting, but one where Extensionality and, crucially, the full Separation scheme fail. Second, we consider structures composed of arbitrary sets using a direct power construction evaluated in a Boolean-valued semantics. Within this setting, we develop the cumulative hierarchy of arbitrary sets, arbitrary ordinals, cardinals, and filtrations of the structure of arbitrary sets. The final section develops a combinatorial interpretation in terms of independent assignments across a fixed state space. Its connection with the iterative conception is then discussed.