Full Idea
Mill's mistake is taking particular applications as integral to the sense of arithmetical propositions. But what is integral to arithmetic is the general principle that explains its applicability, and determines the pattern of particular applications.
Gist of Idea
Mill mistakes particular applications as integral to arithmetic, instead of general patterns
Source
comment on John Stuart Mill (System of Logic [1843], 2.6) by Michael Dummett - Frege philosophy of mathematics Ch.20
Book Reference
Dummett,Michael: 'Frege: philosophy of mathematics' [Duckworth 1991], p.258
A Reaction
[Dummett is summarising Frege's view] Sounds like a tidy objection, but you still have to connect the general principles and patterns to the physical world. 'Structure' could be the magic word to achieve this.