Full Idea
Neither a flock of birds nor a pack of wolves is strictly a set, since a flock can fly south, and a pack can be on the prowl, whereas sets go nowhere and menace no one.
Gist of Idea
A flock of birds is not a set, because a set cannot go anywhere
Source
James Robert Brown (Philosophy of Mathematics [1999], Ch. 7)
Book Reference
Brown,James Robert: 'Philosophy of Mathematics' [Routledge 2002], p.97
A Reaction
To say that the pack menaced you would presumably be to commit the fallacy of composition. Doesn't the number 64 have properties which its set-theoretic elements (whatever we decide they are) will lack?