 The Poincaré-Russell diagnosis of this argument states that this definition does not pick out a collection at all: it is impossible to define a collection S by a condition that implicitly refers to S itself. This is called the vicious circle principle. Definitions that violate the vicious circle principle are called impredicative. A sound definition of a collection only refers to entities that exist independently from the defined collection. Such definitions are called predicative. As Gödel later pointed out, a platonist would find this line of reasoning unconvincing. If mathematical collections exist independently of the act of defining, then it is not immediately clear why there could not be collections that can only be defined impredicatively (Gödel 1944).