Full Idea
In present-day mathematics, it is set theory that serves as the background theory in which other branches of mathematics are developed.
Gist of Idea
Set theory is the standard background for modern mathematics
Source
John P. Burgess (Review of Chihara 'Struct. Accnt of Maths' [2005], §1)
A Reaction
[He cites Bourbaki as an authority for this] See Benacerraf for a famous difficulty here, when you actually try to derive an ontology from the mathematicians' working practices.