The Commens Dictionary
Quote from ‘Notes on Topical Geometry’
A collection is not a thing, but an ens rationis, since its distinctive identity is constituted not only by an arbitrary act, but by the distinctive identities of other things, namely of those that it comprises. At the same time, it so far partakes of the nature of a thing, that it is constituted not at all by anything of a general nature but by individual things, irrespective of their characters. This gives the collection individuality.
A collection is not a thing, and therefore a collection which comprises a single thing and excludes everything else is not identical with that thing. Whatever things may be given, there is just one collection which comprises them all and nothing else. According to this rule, there is a collection which comprises nothing. This collection subsists in every universe.