Full Idea
The distinction between sentences and the abstract propositions that they express is one of the key ideas of logic. A logical argument consists of propositions, assembled together in a systematic fashion.
Gist of Idea
The distinction between sentences and abstract propositions is crucial in logic
Source
Keith Devlin (Goodbye Descartes [1997], Ch. 2)
Book Reference
Devlin,Keith: 'Goodbye Descartes: the end of logic' [Wiley 1997], p.27
A Reaction
He may claim that arguments consist of abstract propositions, but they always get expressed in sentences. However, the whole idea of logical form implies the existence of propositions - there is something which a messy sentence 'really' says.