Full Idea
The fact asserted in the definition of a number is a physical fact. Each of the numbers two, three, four denotes physical phenomena, and connotes a physical property of those phenomena. Two denotes all pairs of things, and twelve all dozens.
Gist of Idea
Numbers denote physical properties of physical phenomena
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.150
A Reaction
The least plausible part of Mill's thesis. Is the fact that a pair of things is fewer than five things also a property? You see two boots, or you see a pair of boots, depending partly on you. Is pure two a visible property? Courage and an onion?