Besides the grammatical form of a language, Humboldt recognized another which he called its _internal form_. This is that subtle something not expressed in words, which even more than the formal parts of speech, reveals the linguistic genius of a nation. It may be defined as the impression which the language bears of the clearness of the conceptions of those speaking it, and of their native gift of speech. He illustrates it by instancing the absence of a developed mode in Sanscrit, and maintains that in the creators of that tongue the conception of modality was never truly felt and distinguished from tense. In this respect its inner form was greatly inferior to the Greek, in the mind of which nation the ideally perfect construction of the verb unfolded itself with far more clearness.
The study of this inner form of a language belongs to the highest realm of linguistic investigation, and is that which throws the most light on the national character and capacities.[17-*]
Humboldt's one criterion of a language was its tendncy[TN-3] to _quicken and stimulate mental action_. He maintained that this is secured just in proportion as the grammatical structure favors clear definition of the individual idea apart from its relations, in other words, as it separates the material from the inflectional elements of speech. Clear thinking, he argued, means progressive thinking. Therefore he assigned a lower position both to those tongues which inseparably connect the idea with its relations, as the American languages, and to those which, like the Chinese and in a less degree the modern English, have scarcely any formal elements at all, but depend upon the position of words (placement) to signify their relations.
But he greatly modified this unfavorable judgment by several extenuating considerations.
Thus he warns us that it is of importance to recognize fully "that grammatical principles dwell rather in the mind of the speaker than in the material and mechanism of his language."[17-+]
This led him to establish a distinction between _explicit_ grammar, where the relations are fully expressed in speech, and _implicit_ grammar, where they are wholly or in part left to be understood by the mind.