Here's a brief, yet entirely authoritative post, on the complicated issues surrounding Reuven's loss of his firstborn status. It starts, as most things do, with a verse in Genesis.
Israel moved on again and pitched his tent beyond Migdal Eder. While Israel was living in that region, Reuben went in and slept with his father’s concubine Bilhah, and Israel heard of it. Now the sons of Jacob were twelve
Modern interpreters claim the verse in italics seems like an insertion. Their grounds for this claim is that the verse has nothing to do with the surrounding passages and that very little is said about the episode. There's no dialog, for instance, and no details about the nature of the sin itself are provided. It's all very brief - unusually brief. The reason for this, suggest the moderns, is that the line about Reuven's sin was a later addition to an existing story.
And what prompted the insertion? Another verse in Genesis where Jacob disqualifies his son on the grounds "that he went up to his father's bed and defiled it." (Gen 49:4) According to the modern interpreters, the blessings belong to an older strata of scripture, and were part of the earliest collection of Israelite literature. When the time came to surround the cherished older pieces with narratives, a lacuna was noticed.
Imagine, for a moment the verse described as an insertion hasn't yet been added. An Israelite reading over Jacob's blessings would have some obvious questions: When did Ruben do the terrible things ascribed to him? When had he ever defiled his father's couch, and with who? What did Ruben do to justify such a bed-side chewing out?
If the story that answered these questions was known, but not represented in the existing materials, perhaps the redactor solved the problem by inserting a brief mention of the tale everyone knew. If no such story existed, perhaps he solved the problems raised by Jacob's blessing by inventing the story. In either case, say the moderns, we have enough reason to believe that it was added at some late date, expressly to solve a problem created when the texts were combined.
This claim rests on the assumption that an insertion needs to be extremely brief, because the person doing the insertion has too much respect for his material to change it more than is absolutely necessary. I'm not sure I buy that. Why would the person making the change necessarily have any special respect for his material? He's changing it after all. Once you're allowing for the possibility that changes were made, what rules out the possibility that those changes were extensive?
-- End interpolation --
The explanation provided by moderns for the original nature of Jacob's blessing to Reuben is also intriguing. As those of you who study Nach may have noticed, no one from the the tribe of Ruben appears after Judges, and the tribe itself is only mentioned as one of the three that settled the far side of the Jordan. There are no Reuben hero or villains, and no trips through Reubenite territory. It seems, as some scholars have noted, that by the beginning of the First Temple Reuben had just about disappeared. If we accept the scholarly premise that Moshe's blessings in Deuteronomy date to around the same period, the reality of Reuben's decline may be reflected when Moshe blesses Ruben with the words: "Let Reuven live and not die" (Deut 33:6) Moreover, the argument continues, perhaps the legend of Reuven's loss of the firstborn status was originally needed to explain that decline. If once-powerful Reuven was nearly gone some explanation for this demise was needed. The story of a sin committed by the namesake ancestor, may have been created to provide it.
(Something similar seems to have occurred with Simon. The tribe is also nearly invisible in Nach, and unlike Reuben, Simon is omitted entirely from Moshe's final blessing. Perhaps this is because the tribe had already vanished? )
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