Full Idea
We consider the meanings of words in isolation, which leads us to accept an idea as the meaning, and words with no mental picture appear to have no mental content. But only in a proposition have the words really a meaning.
Gist of Idea
Words in isolation seem to have ideas as meanings, but words have meaning in propositions
Source
Gottlob Frege (Grundlagen der Arithmetik (Foundations) [1884], §60)
Book Reference
Frege,Gottlob: 'The Foundations of Arithmetic (Austin)', ed/tr. Austin,J.L. [Blackwell 1980], p.71
A Reaction
Frege (later) sees concepts as functions, which need input and output to be understood. It points to the idea that meaning is nothing more than usage. Something, though, is missing. As ever, WHY does something have a particular function?