Raymond Brown traces the Sign Source theory to Rudolf Bultmann who propose that the Fourth Gospel was composed of three different sources in response to perceived problems with the unity of the composition:
- Differences in the style of Greek used in different portions of the text (particular in the Prologue and in Chapter 21).
- Breaks or inconsistencies in sequence
- Repetitions of text (e.g. cf 5:19-25, 5:26-30)
The Signs Source is proposed on the basis of the second of these. There are two statements early in the gospel.
First in John 2:11 (ESV):
This, the first of his signs, Jesus did at Cana in Galilee, and manifested his glory. And his disciples believed in him.
And then in John 4:54:
This was now the second sign that Jesus did when he had come from Judea to Galilee.
However between these two verses in John 2:23 we read:
Now when he was in Jerusalem at the Passover Feast, many believed in his name when they saw the signs that he was doing.
This would seem to indicate that Jesus has already performed signs plural before we get to his "second sign" in 4:54. Bultmann concluded that these passages together with others (e.g. 12:37, 20:30) were indications of a separate written Sign Source - an earlier written document that contained these passages and others - that the editor of the Fourth Gospel must have used in his construction of the final form. Bultmann proceded to try to identify these passages on the basis of the Greek style which was supposed to betray "strong Semitic affinities (verb before subject, absence of connective particles, etc.)." Another proposed indicator of the source was its agenda: "It was composed among Christians converted from among followers of JBap and was originally used as a mission document to persuade their former confreres to follow Jesus." A full reconstructed text can be found in Dwight Moody Smith's, "The Composition and Order of the Fourth Gospel."
In the eighties gospel studies shifted away from form and source criticism towards a study of the gospels in their final form. In 1982, Rhoads and Michie published "Mark as Story." Kingsburry published "Matthew as Story" in 1986. And R. Alan Culpepper published his work, "Anatomy of the Fourth Gospel", in 1983. These works attempted to read the final forms of the gospels as a unified and coherent narrative.
Of course it remained to be seen whether they could be read as coherent narratives. Writing in his preface to the paperback print edition in 1987, Culpepper seems to believe that the program has, in fact, succeeded:
Especially important for this project was the question: Is John sufficiently coherent as narrative to sustain such a study? Whereas earlier Johannine studies often emphasized the discontinuities in John and postulated the use of sources drawn from different settings, I believe Anatomy of the Fourth Gospel demonstrated that the Fourth Gospel develops narration, themes, characterization, ironies, and symbolism with a great deal of internal consistency.
Between Culpepper's work and others following in the same footsteps, there has been a good bit of work re-evaluating particularly the breaks/seams and repetitions that Bultmann considered problematic. This further consideration has led some to conclude that in many cases these artifacts actually contribute to the unity and coherence of the narrative. For instance, whereas before the verses above (2:11 and 4:54) were seen as the start of an enumeration that drops off, now they are seen by some as forming an inclusio around what they refer to as the Cana Cycle.
I don't know if anyone would say that Bultmann's theory has been refuted, but it seems less relevant given the current state of study. Some have instead adopted a multi-stage approach to composition, proposing a core set of signs and discourses that then was edited and embellished over time at later stages of editing. While others - still recognizing there are signs of editorial work in the text - are skeptical that a reconstruction of the editorial process is possible with any much certainty. Andrew Lincoln in his commentary writes, "Detecting signs of editorial work is one thing; reconstructing the process of composition with any precision is another. Even the attempt to sketch what might be its main stages involves inevitable guesswork."
Brown, R. E., & Moloney, F. J. (2003). An Introduction to the Gospel of John (p. 48). New York: Doubleday."
Culpepper, R. A. (1987). Anatomy of the Fourth Gospel: A Study in Literary Design (p. ix). Philadelphia: Fortress Press.
Lincoln, A. T. (2005). The Gospel according to Saint John (p. 50). London: Continuum.