Full Idea
A pair of diamonds is merely an entity of reason, and even if one of them is brought close to another, it is an entity of imagination or perception, that is to say a phenomenon; contiguity, common movement and the same end don't make substantial unity.
Gist of Idea
We find unity in reason, and unity in perception, but these are not true unity
Source
Gottfried Leibniz (Letters to Antoine Arnauld [1686], 1687.04.30), quoted by Daniel Garber - Leibniz:Body,Substance,Monad 7
Book Reference
Garber,Daniel: 'Leibniz: Body, Substance, Monad' [OUP 2009], p.295
A Reaction
This invites the question of what you have to do to two objects to give them substantial unity. The distinction between unity 'of reason' and unity 'of perception' is good.