The paper deals with a long-standing problem of Slavic Historical Linguistics, which is the question why in the history of Old East Slavic (OESl) there occurred a substitution of the Adjective by the Genitive case of a noun as a means of expressing possession. The main postulate is that there existed a previously unnoticed connection between this substitution, on the one hand, and the rise of a new syncretic Accusative case form, known as Genitive-Accusative, on the other. It is suggested that the missing link underlying both processes is the intolerance of the language to Subject-Object ambiguity.
The conclusions are drawn on the basis of the data from a wide range of Medieval East Slavic original sources. The class of nouns involved includes masc. sg. animate nouns (most notably, proper names) belonging to a major declension, the stem of which typically formed a possessive Adjective. The Adjective was practically the only means to express possession in noun phrases denoting mature male individuals - prototypical Possessors. Attestations of the Genitive of possession among this class of nouns in OESl are extremely scarce. Such attestations gradually increased historically, resulting in a completely reverse situation to that of modern Russian, with its exclusive use of the Genitive of possession.
The most important fact concerning the data is that the two alternative morphosyntactic sets: Adjective vs. Genitive in possessive constructions, on the one hand, and Nominative-Accusative vs. Genitive-Accusative on the other, are attested in variation. Upon a detailed and multi-dimensional analysis of the factors influencing the choice of a particular construction, it becomes evident that both sets are sensitive to the same collection of features. Moreover, the relevant features are differently ranked and thus comprise a hierarchy. Since the features on the top of the hierarchy are most typically associated with nouns denoting agents/possessors, it can be termed an “Agent/Possessor Hierarchy”.
Because of the observed correlation between prototypical Agent (Subject) of the transitive action and prototypical Possessor, the Subject-Object distinction at the centre of the Genitive-Accusative debate emerges as not only relevant for the rise of the new Accusative case form, but also for the form of the attributive modifier. This underlies the claim that the hierarchy of features regulates both phenomena in morphosyntactic variation. The arrangement of shared features relevant for both sets of constructions into a hierarchy also explains why these two changes, documented at their initial stage by the OESl texts, occurred parallel to each other. The correlation of Subject with the Adjective and Object with the Genitive case of a noun may have been influenced by the same factors, as was Genitive-Accusative syncretism. Since proper personal stems representing prototypical Agents/Possessors predominantly formed possessive Adjectives, Genitive case of such nouns was not widely employed in possessive constructions. One can speculate then, that Genitive was available for the use as Object of transitive verbs with this specific class of nouns. Researchers, stressing the decline in the Genitive direct objects, have given no consideration to the unproductive Genitive usage in possessive constructions among nouns high on the hierarchy. The distribution pattern in possessive constructions could have contributed to the new productivity of the Genitive case and ultimately to the rise of the Genitive-Accusative.