Gödel’s conjecture: every set has a defining concept
Establish whether for every set there exists a defining concept in the envisaged logic of concepts whose extension is exactly that set, thereby enabling set theory to be encompassed within a hierarchy of concepts.
References
G¨odel remarks that the future logic of concepts might contain set theory. This is explained by his conjecture that for every set, there is a (defining) concept ([22], 8.6.4).
— Kurt Gödel and the Logic of Concepts
(2406.05442 - Kostić et al., 8 Jun 2024) in Section 7.2