Full Idea
Zermelo assumes that not every predicate has an extension but rather that given a set we may separate out from it those of its members satisfying the predicate. This is called 'separation' (Aussonderung).
Gist of Idea
Not every predicate has an extension, but Separation picks the members that satisfy a predicate
Source
report of Ernst Zermelo (Investigations in the Foundations of Set Theory I [1908]) by William D. Hart - The Evolution of Logic 3
Book Reference
Hart,W.D.: 'The Evolution of Logic' [CUP 2010], p.69
Related Idea
Idea 18105 Replacement enforces a 'limitation of size' test for the existence of sets [Bostock]