Whatever were the difficulties, therefore, which embarrassed the first invention of nouns adjective, the same, and many more, must have embarrassed that of prepositions. If mankind, therefore, in the first formation of languages, seem to have, for some time, evaded the necessity of nouns adjective, by varying the termination of the names of substances, according as these varied in some of their most important qualities, they would much more find themselves under the necessity of evading, by some similar contrivance, the yet more difficult invention of prepositions. The different cases in the ancient languages is a contrivance of precisely the same kind. The genitive and dative cases, in Greek and Latin, evidently supply the place of the prepositions; and by a variation in the noun substantive, which stands for the co-relative term, express the relation which subsists between what is denoted by that noun substantive, and what is expressed by some other word in the sentence. In these expressions, for example, _fructus arboris_, _the fruit of the tree_; _sacer Herculi_, _sacred to Hercules_; the variations made in the co-relative words, _arbor_ and _Hercules_, express the same relations which are expressed in English by the prepositions _of_ and _to_. To express a relation in this manner, did not require any effort of abstraction. It was not here expressed by a peculiar word denoting relation and nothing but relation, but by a variation upon the co-relative term. It was expressed here, as it appears in nature, not as something separated and detached, but as thoroughly mixed and blended with the co-relative object. To express relation in this manner, did not require any effort of generalization. The words _arboris_ and _Herculi_, while they involve in their signification the same relation expressed by the English prepositions _of_ and _to_, are not, like those prepositions, general words, which can be applied to express the same relation between whatever other objects it might be observed to subsist.