Full Idea
Whatever is made up of parts is made up of parts of those parts.
Gist of Idea
Whatever is made up of parts is made up of parts of those parts
Source
John Stuart Mill (System of Logic [1843], 3.24.5)
Book Reference
Mill,John Stuart: 'System of Logic (9th ed, 2 vols)' [Longmans, Green etc 1875], p.153
A Reaction
Mill considers this principle to be fundamental to the possibilities of arithmetic. Presumably he thought of it as an inductive inference from our dealings with physical objects.