A verb, in the most restricted meaning of the term, is that part of speech, by which a subject is connected with its attribute. According to this definition it would appear, that there can exist only one verb, namely, the substantive verb, in Latin esse; in English, to be. But even these are sometimes used as attributive verbs […]. This verb is more generally used as a mere grammatical copula, without conveying the idea of existence […]. It appears to me, that it is from the want of a mere grammatical copula, that languages make use for this purpose of a verb already attributive; but among attributive verbs, to be is certainly the most convenient, because every subject, to which an attribute is ascribed, exists, or is at least supposed to exist. An attribute which may be expressed by an adjective can be included in the verb itself, and such attributive verbs incline more towards the nature of adjectives than that of verbs in their grammatical functions. Languages of a structure similar to that of the Greek, Latin &c. can express by one verb of this kind a whole logical proposition, in which, however, that part of speech which expresses the connexion of the subject with its attribute, which is the characteristic function of the verb, is generally entirely omitted or understood.
- Bopp (1820), Pag. 13-14