Full Idea
It is sufficient to observe that all unities are propositions or propositional concepts, and that consequently nothing that exists is a unity. If, therefore, it is maintained that things are unities, we must reply that no things exist.
Gist of Idea
Unities are only in propositions or concepts, and nothing that exists has unity
Source
Bertrand Russell (The Principles of Mathematics [1903], §439)
Book Reference
Russell,Bertrand: 'Principles of Mathematics' [Routledge 1992], p.467
A Reaction
The point, I presume, is that you end up as a nihilist about identities (like van Inwagen and Merricks) by mistakenly thinking (as Aristotle and Leibniz did) that everything that exists needs to have something called 'unity'.