Scholar Richard L. Schultz comments on the subjectivity in analyzing parallels between biblical passages.
Richard L. Schultz, The Search for Quotation: Verbal Parallels in the Prophets (Journal for the Study of the Old Testament Supplement Series vo.. 180; Sheffield: Sheffield Academic Press, 1999), 58
The methodological problem confronting scholars has three major components:
Identifying Quotations. It is by no means the case that every striking verbal parallel was automatically labeled a 'quotation'. Rather than attributing all similarities in wording to one prophet consciously citing another, scholars have proposed numerous alternative explanations-coincidence, unconscious imitation, divine inspiration, formulaic, proverbial or cultic language, oral transmission, mutual dependence on unpreserved material, similarity of background and circumstances, redactional glosses. Although some of the alternative explanations no longer may be considered viable options and not of them would apply equally well to a given passage, nevertheless, progress has been made in determining what influences might have produced a verbal parallel if it is not a quotation. However, completely reliable criteria for identifying what is (or even may be) genuine quotation have yet to be discovered. Part of the problem is terminological: Is 'quotation' clearly synonymous with 'literary borrowing' or 'conscious imitation'? The other part is syntactical: given the absence of clear indicators, such as introductory formulae or quotation marks, determining dependence remains little more than an educated guess.
Assessing the nature of the borrowing. Several centuries ago, a verbal parallel between the books of Ezekiel and Isaiah might be assessed simply as a case of Ezekiel quoting Isaiah. However, today the situation is understood as being considerably more complex. In the light of subsequent developments in the critical study of the prophetic books, one might consider all of the following alternatives: Ezekiel is quoting (1) the words of Isaiah the eighth-century prophet; (2) an edited collection of Isaiah's oracles; (3) a later prophet of the 'Isaiah school'; or (4) a saying originated by Isaiah which by Ezekiel's day had become formulaic or proverbial. Alternatively, the parallel in Ezekiel could be (5) a post-Ezekiel redactional addition; or (6) even stem from a later prophet or editor within the Isaiah 'tradition' who is quoting Ezekiel.
Determining the direction of borrowing. Before the rise of critical studies, it was a simple matter to determine who was borrowing from whom, except in cases of contemporaries such as Isaiah and Micah and prophets such as Habakkuk, Nahum and Joel who historical background is difficult to determine. A new critical consensus regarding the dating of some of the prophetic literature has emerged, once again encouraging a priori determinations of the direction of dependence. However, with regard to contemporaries and prophetic literature whose date is still being debated and as a means of testing again critically determined chronologies, criteria are still essential for ascertaining who the borrower is. As the polemics of the late nineteenth-century illustrate, most criteria are extremely flexible, able to be used persuasively by either side: did the quoter expand or abbreviate borrowed material? Did he gather together multiple fragments from his predecessor and build them into a new composite oracle or did he break up the longer saying of a predecessor into seed-thoughts to be scattered wherever appropriate through his oracles? Did he carefully adapt a borrowed saying so that it fitted more perfectly into its new setting than in its original location or is an awkward contextual fit evidence that he sought to preserve the saying exactly as he found it? Although such criteria are essential, their application is almost unavoidably subjective.