Full Idea
Cartesian 'ideas' seem to be both concepts and propositions at once.
Gist of Idea
Cartesian 'ideas' confuse concepts and propositions
Source
Roger Scruton (Short History of Modern Philosophy [1981], Ch.4)
Book Reference
Scruton,Roger: 'A Short History of Modern Philosophy' [ARK 1985], p.44
A Reaction
This seems to be the simple reason why modern philosophers don't like this seventeenth century notion. There is something slightly too tidy about the modern notion of propositions built out of concepts. Animals see propositions in a flash.