Vygotsky – Thought and Language

“The adolescent will form and use a concept quite correctly in a concrete situation but will find it strangely difficult to express that concept in words, and the verbal definition will, in most cases, be much narrower than might have been expected from the way he used the concept. The same discrepancy occurs also in adult thinking, even at very advanced levels. This confirms the assumption that concepts evolve in ways differing from deliberate conscious elaboration of experience in logical terms. Analysis of reality with the help of concepts precedes analysis of the concepts themselves.”
— An Experimental Study of Concept Formation

“Our investigation has shown that the development of writing does not repeat the developmental history of speaking. Written speech is a separate linguistic function, differing from oral speech in both structure and mode of functioning. Even its minimal development requires a high level of abstraction. It is speech in thought and image only, lacking the musical, expressive, intonational qualities of oral speech. In learning to write, the child must disengage himself from the sensory aspect of speech and replace words by images of words. Speech that is merely imagined and that requires symbolization of the sound image in written signs (i.e. a second degree of symbolization) naturally must be as much harder than oral speech for the child as algebra is harder than arithmetic. Our studies show that it is the abstract quality of written language that is the main stumbling block, not the underdevelopment of small muscles or any other mechanical obstacles.
Writing is also speech without an interlocutor, addressed to an absent or imaginary person or to no one in particular — a situation new and strange to the child. […] In conversation, every sentence is prompted by a motive. Desire or need lead to request, question to answer, bewilderment to explanation. The changing motives of the interlocutors determine at every moment the turn oral speech will take. It does not have to be consciously directed — the dynamic situation takes care of that. The motives for writing are more abstract, more intellectualized, further removed from immediate needs. In written speech, we are obliged to create the situation, to represent it to ourselves. This demands detachment from the actual situation.”
— Development of Scientific Concepts in Childhood

“Our experiments convinced us that inner speech must be regarded, not as speech minus sound, but as an entirely separate speech function. Its main distinguishing trait is its peculiar syntax. compared with external speech, inner speech appears disconnected and incomplete.
[…]
Pure predication [providing information on a subject without specifying subject] occurs in external speech in two cases: either as an answer or when the subject of the sentence is known beforehand to all concerned. the answer to “Would you like a cup of tea?” is never “No, I don’t want a cup of tea,” but a simple “No.” Obviously, such a sentence is possible only because its subject is tacitly understood by both parties. […]
very good examples of the condensation of external speech and its reduction to predicates are found in the movels of Tolstoy, who quite often dealt with the psychology of understanding (Kitty and Levin)”
— Thought and Word