Full Idea
If one cannot think in a language, one has not yet mastered it. A symbol system used only for communication, like Morse code, is not a language.
Gist of Idea
Mastery of a language requires thinking, and not just communication
Source
Gilbert Harman ((Nonsolipsistic) Conceptual Role Semantics [1987], 12.1.2)
Book Reference
Harman,Gilbert: 'Reasoning Meaning and Mind' [OUP 1999], p.208
A Reaction
This invites the question of someone who has mastered thinking, but has no idea how to communicate. No doubt we might construct a machine with something like that ability. I think it might support Harman's claim.