Full Idea
The basic definition of a graph can be given in set-theoretic terms,...but then what could an unlabelled graph be?
Gist of Idea
When graphs are defined set-theoretically, that won't cover unlabelled graphs
Source
James Robert Brown (Philosophy of Mathematics [1999], Ch. 7)
Book Reference
Brown,James Robert: 'Philosophy of Mathematics' [Routledge 2002], p.105
A Reaction
An unlabelled graph will at least need a verbal description for it to have any significance at all. My daily mood-swings look like this....