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Full Idea
Definitions are called 'predicative', and are considered sound, if they only refer to entities which exist independently from the defined collection.
Gist of Idea
Predicative definitions only refer to entities outside the defined collection
Source
Leon Horsten (Philosophy of Mathematics [2007], §2.4)
Book Ref
'Stanford Online Encyclopaedia of Philosophy', ed/tr. Stanford University [plato.stanford.edu], p.8