Full Idea
The unpredicative nature of number words can be seen by noting how different they are from, say, ordinary adjectives, which do function as predicates.
Gist of Idea
Number words are not predicates, as they function very differently from adjectives
Source
Paul Benacerraf (What Numbers Could Not Be [1965], II)
Book Reference
'Philosophy of Mathematics: readings (2nd)', ed/tr. Benacerraf/Putnam [CUP 1983], p.283
A Reaction
He points out that 'x is seventeen' is a rare construction in English, unlike 'x is happy/green/interesting', and that numbers outrank all other adjectives (having to appear first in any string of them).