Full Idea
The only kind of unity to which I can attach any precise sense - apart from the unity of the absolutely simple - is that of a whole composed of parts.
Gist of Idea
The only unities are simples, or wholes composed of parts
Source
Bertrand Russell (The Principles of Mathematics [1903], §439)
Book Reference
Russell,Bertrand: 'Principles of Mathematics' [Routledge 1992], p.466
A Reaction
This comes from a keen student of Leibniz, who was obsessed with unity. Russell leaves unaddressed the question of what turns some parts into a whole.