There's a lot more to a mastery of language than spelling and grammar. Communication is really the key thing. You can communicate articulately and intelligently without having immaculate spelling and grammar. The words you choose, the order you put them in, and your understanding of how to use them in order to convey some thought or expression are what is important. That's why great written works are looked over by an editor prior to publishing. The content and nature of the thought expressed is a lot more important than the mechanics of the language. Or to put it in terms to which Robbo might better relate, Einstein's theory of relativity would be an incredible work of scientific literature even if he spelled "relativity" "r-e-l-i-t-e-v-a-t-y". The meaning of the content isn't really changed by the superficial error in mechanics. In that sense, I think you will find that most "intelligent" people, as we commonly consider the term, have a good mastery of language - in that they are able to effectively convey a thought, concept, or idea. And I do believe there is some natural correlation between the two. The ability to grasp many such thoughts, ideas and concepts is dependent upon the ability to conceptualize things mentally (using the "natural language" of thought) and to understand the mental conceptualizations of others, which requires a medium, such as language, to facilitate. The ability to express a thought, idea, or concept in your own mind almost naturally implies the ability to express it to others as well. For example, can the ability to mentally grasp an abstract concept like "gravity" exist without the ability to logically articulate that concept? And can the ability to logically articulate the concept possibly exist without the ability to express it (in language)?
The reverse, however, is not necessarily true. Unintelligent, or rather less intelligent, people may be able to use language effectively, but as a corollary to the above, the actual thoughts and ideas that the language conveys may be less than impressive. Politicians spring immediately to mind.
The reverse, however, is not necessarily true. Unintelligent, or rather less intelligent, people may be able to use language effectively, but as a corollary to the above, the actual thoughts and ideas that the language conveys may be less than impressive. Politicians spring immediately to mind.