Full Idea
Three pebbles make different sense impressions in one parcel or in two. That the same pebbles by an alteration of place and arrangement may be made to produce either sensation is not the identical proposition.
Gist of Idea
Different parcels made from three pebbles produce different actual sensations
Source
John Stuart Mill (System of Logic [1843], 2.6.2)
Book Reference
Mill,John Stuart: 'System of Logic (9th ed, 2 vols)' [Longmans, Green etc 1875], p.295
A Reaction
[compressed] Not quite clear, but Mill seems to be adamant that we really must experience the separation, and not just think what 'may' happen, so Frege is right that Mill is lucky that everything is not 'nailed down'.