Full Idea
A pack of wolves is not thought to go out of existence just because some member of the pack is killed.
Gist of Idea
A pack of wolves doesn't cease when one member dies
Source
Charles Chihara (A Structural Account of Mathematics [2004], 07.5)
Book Reference
Chihara,Charles: 'A Structural Account of Mathematics' [OUP 2004], p.203
A Reaction
The point is that the formal extensional notion of a set doesn't correspond to our common sense notion of a group or class. Even a highly scientific theory about wolves needs a loose notion of a wolf pack.