Full Idea
Naïve set theory is based on the principles that any formula defines a set, and that coextensive sets are identical.
Gist of Idea
Naïve set theory says any formula defines a set, and coextensive sets are identical
Source
Øystein Linnebo (Philosophy of Mathematics [2017], 4.2)
Book Reference
Linnebo,Øystein: 'Philosophy of Mathematics' [Princeton 2017], p.62
A Reaction
The second principle is a standard axiom of ZFC. The first principle causes the trouble.