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Full Idea
Poincaré suggested that what is wrong with an impredicative definition is that it allows the set defined to alter its composition as more sets are added to the theory.
Gist of Idea
Impredicative definitions are wrong, because they change the set that is being defined?
Source
David Bostock (Philosophy of Mathematics [2009], 8.3)
Book Ref
Bostock,David: 'Philosophy of Mathematics: An Introduction' [Wiley-Blackwell 2009], p.252