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On the same interesting topic:
How useful is case morphology? The loss of the Old French two-case system within a theory of Preferred Argument structure, by Ulrich Detges, In: The Role of Semantic, Pragmatic, and Discourse Factors in the Development of Case Edited by Jóhanna Barðdal and Shobhana L. Chelliah [Studies in Language Companion Series 108] 2009, pp. 93–120.
This paper discusses the loss of the Old French two-case system within the theory of Preferred Argument Structure (Du Bois 2003, 1987). It will be shown that the chronology of this process followed a hierarchy of relative frequency, which in turn was driven by discourse preferences. Apart from strictly historical facts, this explanation also accounts for some basic typological properties of case-marking systems. In particular, it will be argued that no correlation exists between morphological case marking on the one hand and word-order flexibility on the other. Moreover, the theory of Preferred Argument Structure allows for a coherent assessment of the long-standing observation that in many cases, morphological case on full nouns is not necessary for distinguishing subjects from non-subjects.