If the original invention of nouns adjective would be attended with so much difficulty, that of prepositions would be accompanied with yet more. Every preposition, as I have already observed, denotes some relation considered in concrete with the co-relative object. The preposition _above_, for example, denotes the relation of superiority, not in abstract, as it is expressed by the word _superiority_, but in concrete with some co-relative object. In this phrase, for example, _the tree above the cave_, the word _above_, expresses a certain relation between the _tree_ and the _cave_, and it expresses this relation in concrete with the co-relative object, the _cave_. A preposition always requires, in order to complete the sense, some other word to come after it; as may be observed in this particular instance. Now, I say, the original invention of such words would require a yet greater effort of abstraction and generalization, than that of nouns adjective. First of all, a relation is, in itself, a more metaphysical object than a quality. Nobody can be at a loss to explain what is meant by a quality; but few people will find themselves able to express, very distinctly, what is understood by a relation. Qualities are almost always the objects of our external senses; relations never are. No wonder, therefore, that the one set of objects should be so much more comprehensible than the other. Secondly, though prepositions always express the relation which they stand for, in concrete with the co-relative object, they could not have originally been formed without a considerable effort of abstraction. A preposition denotes a relation, and nothing but a relation. But before men could institute a word, which signified a relation, and nothing but a relation, they must have been able, in some measure, to consider this relation abstractedly from the related objects; since the idea of those objects does not, in any respect, enter into the signification of the preposition. The invention of such a word, therefore, must have required a considerable degree of abstraction. Thirdly, a preposition is from its nature a general word, which, from its very first institution, must have been considered as equally applicable to denote any other similar relation. The man who first invented the word _above_, must not only have distinguished, in some measure, the relation of _superiority_ from the objects which were so related, but he must also have distinguished this relation from other relations, such as, from the relation of _inferiority_ denoted by the word _below_, from the relation of _juxtaposition_, expressed by the word _beside_, and the like. He must have conceived this word, therefore, as expressive of a particular sort or species of relation distinct from every other, which could not be done without a considerable effort of comparison and generalization.