I wondered why the authors originally chose to use the genitive of characteristic. Before I read the following, I imagined (given the context now supplied) “Amare est reginae nautaeque” should be translated like “The queen and sailor are destined to be in love”. Now I see (unless I misunderstand) it can mean just “The queen and the sailor are/fall in love”, referring to an internal emotional state, and not necessarily a permanent one.
Me rogavi cur illi auctores istam formulam (per genetivum qualitatis) primitùs scripissent. Ante id legendum quod sequerit, illam formulam rem quae permanet indicare credideram. Quod qualitas affectûs interna et temporaria cogitanda sit nihil obstare nunc (nisi malè intellego) video.
E. C. Woodcock, “Review: Ablative and Genitive of Quality”, The Classical Review, Vol. 61, No. 1 (May, 1947), pp. 22-23 (http://www.jstor.org/stable/704581) Review of Genetivus und Ablativus Qualitatis by Eirik Vandvik
Attention is called to semantic change of value in the various genitive expressions which are usually classed together as Genitives of Quality. V[andvik] would restrict this term to genitive expressions in which the noun denotes a real quality or characteristic (vir maximi anirni, summae audaciae, etc.), and would exclude those in which the noun denotes something external which, with its epithet, serves as a means of classifying (res eius modi, magni periculi res, multi cibi hospes, magni sunt oneris, puer decem annorurm). The latter types are the earlier, and of the former there are only two examples in early Latin. But these classifying expressions, in most of which the possessive idea of ’ belonging to a class’ is fairly clear, were capable of developing a secondary qualitative sense (e.g. magni oneris suggests ‘patience’, ’ endurance’; multi cibi suggests ’ greediness’, etc.). This led to the extension of the construction to nouns denoting internal characteristics and to epithets other than demonstrative or quantitative (homo animi perditi), in which expressions the possessive idea is no longer clear. The bridge between the nouns denoting external things and those denoting internal characteristics is further strengthened by an appeal to the primitive habit of personification. Just as this enabled an external abstract quality to be expressed as an accompaniment by the ablative (femina eximia forma), so an internal quality, such as courage, virtue, vice, when personified into a principle which pervaded, possessed, or governed a person or thing, was naturally expressed by the genitive. V. thus agrees with Edwards and Wolfflin in deriving the Genitive of Quality from the idea of possession, and follows the generally accepted derivation of the Ablative of Quality from the Sociative-Instrumental. But he differs from them in his conclusions about the difference in the underlying sense of the two constructions. It is not that the ablative expresses temporary and the genitive permanent characteristics, but that the ablative originally expressed external, and the genitive internal qualities. The ablative was the normal method of expressing permanent as well as temporary external characteristics from Plautus to Cicero, while the use of the genitive to express external characteristics is poetic and post-Augustan, i.e. cervus vasti corporis is not classical. On the other hand, the extension of the ablative to express internal qualities had already taken place in the time of Plautus. This was due to two main causes: (i) The adverbial origin of the Ablative of Quality caused it to be preferred whenever the quality was predicated, even when the quality was internal. In Cicero 66 per cent. of the Ablatives of Quality are predicative. (2) Authors up to Cicero had some objection to genitives of the third, fourth, and fifth declensions in this construction, and so used the ablatives of them even of internal qualities and adnominally. There is not enough difference between the appearance of a quality, viewed as an accompaniment, and its actual existence apart from appearances, to prevent the use of the ablative to express internal and essential qualities, whenever stylistic or other reasons favoured it…
Authors are divided into two groups: (I), the early dramatic poets, Cicero, Caesar, Nepos, and Sallust; (2), Cicero’s correspondents, the ‘Caesarian’ authors of B.G. viii, Bellum Alex., Hispan., and Afric., Livy,. and the authors of the Silver Age. Tacitus, with a finer linguistic sense than other authors of his age, shows a tendency to return to classical usage, and occupies a position midway between the two groups. The authors of the first group, particularly Cicero and Caesar, were keenly alive to the distinctions between the two constructions, which are indicated above. In them apparent disregard of the inner sense can be traced to observable reasons, semantic or stylistic. But their reasons were not appreciated by the authors who followed them. These began to prefer the genitive for both external and internal qualities, both predicatively and adnominally; genitives of the third and fourth declensions were no longer banned, etc., and from Livy onwards the distinction in sense between the ablative and genitive in these expressions was clearly no longer felt…
It is probable that the findings of this investigation, as often happens, are too complicated to be fully expounded in text-books intended for schools and colleges, but present methods of exposition must be modified in the light of them.