Hence, in Humboldt’s theory the psychological element of _self-consciousness_ lies at the root of all linguistic expression. No mere physical difference between the lower animals and man explains the latter’s possession of articulate speech. His self-consciousness alone is that trait which has rendered such a possession possible.[10-†] The idea of Self necessarily implies the idea of Other. A thought is never separate, never isolated, but ever in relation to another thought, suggested by one, leading on to another. Hence, Humboldt says: “The mind can only be conceived as in action, and _as action_.” As Prof. Adler, in his comments on Humboldt’s philosophy, admirably observes: “Man does not possess any such thing as an absolutely isolated individuality; the ‘I’ and the ‘thou’ are the essential complements of each other, and would, in their last analysis, be found identical.”[11-*] On these two fundamental conceptions, those of Identity and Relation, or, as they may be expressed more correctly, those of Being and Action, Humboldt builds his doctrines concerning the primitive radicals of language and the fundamental categories of grammar. § 6. PRIMITIVE ROOTS AND GRAMMATICAL CATEGORIES. The roots of a language are classified by Humboldt as either _objective_ or _subjective_, although he considers this far from an exhaustive scheme.[11-†] The objective roots are usually descriptive, and indicate an origin from a process of mental analysis. They bear the impress of those two attributes which characterize every thought, Being and Action. Every complete objective word must express these two notions. Upon them are founded the fundamental grammatical categories of the Noun and the Verb; or to speak more accurately, they lead to the distinction of nominal and verbal themes. The characteristic of the Noun is that it expresses Being; of the Verb that it expresses Action. This distinction is far from absolute in the word itself; in many languages, especially in Chinese and some American languages, there is in the word no discrimination between its verbal and nominal forms; but the verbal or nominal _value_ of the word is clearly fixed by other means.[11-‡] Another class of objective root-words are the adjective words, or Determinatives. They are a later accession to the list, and by their addition bring the three chief grammatical categories, the Noun, the Verb and the Adjective, into correlation with the three logical categories of Substance, Action and Quality. By the subjective roots, Humboldt meant the personal pronouns. To these he attributed great importance in the development of language, and especially of American languages. They carry with them the mark of sharp individuality, and express in its highest reality the notion of Being.