Mr. Zermelo does not allow himself to consider the set of all the objects which satisfy a certain condition because it seems to him that this set is never closed; that it will always be possible to introduce new objects. On the other hand, he has no scruple in speaking of the set of objects which are part of a certain Menge M and which also satisfy a certain condition. It seems to him that one cannot possess a Menge without possessing at the same time all its elements. Among these elements, he will choose those which satisfy a given condition, and will be able to make this choice very calmly, without fear of being disturbed by the introduction of new and unforeseen elements, since he already has all these elements in his hands. By positing beforehand this Menge M, he has erected an enclosing wall which keeps out the intruders who could come from without. But he does not query whether there could be intruders from within whom he enclosed inside his wall. (Poincaré 1909: 477; p. 59 of the English translation)