Full Idea
The transition from using number words purely as adjectives to using them extensively as nouns has been traced to 'around the time of Plato'.
Gist of Idea
Number words became nouns around the time of Plato
Source
JP Burgess / G Rosen (A Subject with No Object [1997], III.C.2.a)
Book Reference
Burgess,J/Rosen,G: 'A Subject with No Object' [OUP 1997], p.228
A Reaction
[The cite Kneale and Kneale VI,§2 for this] It is just too tempting to think that in fact Plato (and early Platonists) were totally responsible for this shift, since the whole reification of numbers seems to be inherently platonist.