Full Idea
Hard-headed realism tends to embrace the full Comprehension Principle, that every well-defined concept determines a set.
Gist of Idea
Realisms like the full Comprehension Principle, that all good concepts determine sets
Source
Stephen Read (Thinking About Logic [1995], Ch.8)
Book Reference
Read,Stephen: 'Thinking About Logic' [OUP 1995], p.214
A Reaction
This sort of thing gets you into trouble with Russell's paradox (though that is presumably meant to be excluded somehow by 'well-defined'). There are lots of diluted Comprehension Principles.