Full Idea
Everything is surely related to everything as follows: either it is the same or different; or, if it is not the same or different, it would be related as part to whole or as whole to part.
Gist of Idea
Two things relate either as same or different, or part of a whole, or the whole of the part
Source
Plato (Parmenides [c.366 BCE], 146b)
Book Reference
Plato: 'Complete Works', ed/tr. Cooper,John M. [Hackett 1997], p.379
A Reaction
This strikes me as a really helpful first step in trying to analyse the nature of identity. Two things are either two or (actually) one, or related mereologically.