But this psychical state may be studied otherwise than by internal observation, and the subsequent interpretation. It lends itself to an _objective_ study, because it is incarnated in certain words. When I say, red _and_ green, red _or_ green, there are in either case, not two, but _three_ states of consciousness; the sole difference is in the intermediate state which corresponds with an inclusion or an exclusion. So, too, all our prepositions and conjunctions (_for_, _by_, _if_, _but_, _because_) envelop a mental state, however attenuated. The study of languages teaches us that the expression of relations is produced in two ways, forming, as it were, two chronological layers. The most ancient is that of the cases or declensions: a highly complex mechanism, varying in marked degree with the idioms, and consisting in appositions, suffixes, or modifications of the principal theme. But these relations have only acquired their proper linguistic organ, specialised for this function, by means of prepositions and conjunctions. They are wanting in many languages; gesture being then substituted for them. The principal parts of the discourse are solitary, juxtaposed without links after the manner of the phrases used by children. Others, somewhat less poor, have only two conjunctions: _and_, _but_. In short, the terms on which devolved the expression of relations are of late formation, as it were, organs _de luxe_. In the analytical languages, prepositions and conjunctions are nouns or pronouns diverted from their primitive acceptation, which have acquired a value expressive of transition, condition, subordination, co-ordination, and the rest. The psychological notion common to the greater number, if not to all, is that of a movement. “All relations expressed by prepositions can be referred to repose, and to movement in space and time, i. e., to those with which the locative, accusative (movement of approximation) and ablative (movement of departure) correspond in declension.”[65] It may be admitted that this consciousness of movement, of change, which is no more, fundamentally, than the sense of different directions of thought, belongs less to the category of clear notions than to that of subconscious states, of tendencies, of actions, which explains why the terms of relation are wholly wanting, or rare, and only conquered their autonomy at a late period. With these, the progressive work of differentiation is accomplished. Discourse has now its materials and its cement; it is capable of complex phrases wherein all is referred and subordinated to a principal state, contrary to those ruder essays which could only attain to simple phrases, denuded of connective apparatus.