than the suggestion of the same relation by a case form, 
     but it is correct to say that the relation can become 
     associated with a single word only when it is felt with a 
     considerable degree of clearness. The relation between 
     concepts must itself become a concept. To this extent 
     the movement toward the expression of relation by single  
     words is a movement toward precision . . . . The adverb-
     preposition is the expression in more distinct form of 
     some element of meaning which was latent in the case-
     form. It serves therefore as a definition of the meaning of 
     the case-form. (pp. 102­4)