Full Idea
What is the union of the singleton {0}, of zero, and the singleton {φ}, of the null set? Is it the one-element set {0}, or the two-element set {0, φ}? Unless the question of identity between 0 and φ is resolved, we cannot say.
Gist of Idea
Unless we know whether 0 is identical with the null set, we create confusions
Source
Kit Fine (Replies on 'Limits of Abstraction' [2005], 2)
Book Reference
-: 'Philosophical Studies' [-], p.388