When two texts share the same language or tell the same scene, the overlap by itself does not explain how they are connected but shows only that a connection of some kind is likely. Working out what that connection is takes more than noticing the resemblance, and the answer is often not the obvious one.
The connection can take one of three basic forms, since the first text may have used the second, the second may have used the first, or both may have drawn on an earlier source that one or both of them no longer preserve. A fourth option, that the likeness is pure coincidence, can usually be set aside once the agreement is close enough. Each of the three real options carries a different meaning, and telling them apart is one of the harder tasks in reading texts next to each other.
One Text Used the Other
The simplest relationship is direct dependence, where one writer had the other's text and used it. The sign of it is language that agrees too closely, and too often, to be explained by chance or by two people describing the same thing in their own words. Once that kind of agreement turns up, the connection is real, and the harder question becomes direction, which text came first and which one used it.
Direction is not settled by the order of the books in the Bible, and it is not settled by the date a book reached its final shape, since a late book can carry old material and an early-looking one can be revised over a long period. It has to be argued from the texts themselves. Common signs include one writer smoothing out the other's awkward phrasing, dropping a quotation the other had included, or expanding a point the other had stated briefly.
One useful test works from a kind of slip. A writer who is reworking a source will sometimes change its language at the start of a passage and then, further along, fall back into the source's exact words, which gives away which text was being copied. The first three Gospels show this at work, since in the material all three share, Matthew and Luke each begin by improving on Mark's rough Greek and then drift back into his phrasing partway through a story, which is far easier to explain if they were working from Mark than if Mark was working from them.[1] The reverse slip, the one that would be needed to argue the other direction, is not there.
The letters of Jude and 2 Peter are a good case. Their content overlaps so heavily that almost no one treats the likeness as accidental, and the standard view is that one letter used the other. The weight of opinion, together with the internal signs, points to the author of 2 Peter using Jude rather than the reverse.[2] 2 Peter tends to smooth over language that reads awkwardly in Jude, and it leaves out Jude's direct quotation of 1 Enoch, which is easier to explain as 2 Peter trimming a source than as Jude adding one.[3] The resemblance was the easy part, and working out the direction took a close look at which writer had edited the other.
Both Used a Shared Source
Sometimes two texts agree closely, yet neither shows the signs of having used the other. In that case a shared source both of them drew on can account for the overlap, even when that source has not survived and is known only by working backward from the texts that used it.
The first three Gospels raise the question again. Beyond the material all three have in common, which Matthew and Luke drew from Mark, there is a large body of material that Matthew and Luke share but that is missing from Mark, often in nearly the same words.
One long-standing explanation is that Matthew and Luke each drew on a lost collection of Jesus's sayings, labeled Q, without either one using the other.[4] The explanation is not settled, since others argue that Luke had Matthew in front of him and used it directly, so no lost source is needed, and that the agreement between the two is closer than a shared source would produce.[5] The same shared language is read by one side as evidence of a common source and by the other as evidence of direct dependence, which shows how hard it is to tell the two apart.
A third explanation sets written sources aside for this shared material. On this view the agreements go back to the spoken tradition about Jesus that circulated in the earliest communities, carried carefully enough that two writers drawing on it could end up close to each other without either one using a written source.[6] That account moves the shared material out of the first two options and into the third, where the common ground is a living tradition rather than a document.
The Resemblance Comes from a Shared Tradition
The third possibility is that the shared material was never a single text at all. A story, a set of images, or a standard way of telling an event can circulate widely across a culture, so two writers can end up with similar material without either one reading the other and without any single document behind them both.
The flood story is a clear case, since versions of it were told across Mesopotamia for centuries, in the Atrahasis epic, in a Sumerian flood account, and in the eleventh tablet of the Gilgamesh epic, where the flood was added to a poem that had first circulated without it. The flood in Genesis is recognizably a Judean telling of this widespread Mesopotamian flood tradition rather than a copy of any one tablet, while the material around it in the opening chapters of Genesis matches the general themes of Mesopotamian accounts of early human history, such as long-lived ancestors and the origins of cities and crafts, without following the plot of any single Mesopotamian text.[7] That pattern, general agreement without a matching storyline, is the strongest sign that a resemblance comes from a shared tradition rather than a specific text that one author copied.
Telling the Cases Apart
Because the three relationships look alike on the surface, readers have tried to set out tests for deciding when a connection is real and what kind it is. The common measures include whether the earlier text was available to the later writer, how much language the two share, how distinctive it is, and whether the supposed link fits the later writer's known interests.[8] None of these settles a case on its own, and the same evidence can be read more than one way, which is why proposed lists of tests are argued over as much as the cases they are meant to decide.
The measures still point in a usable direction. Close agreement in distinctive language, especially phrasing that is unusual or hard to arrive at on one's own, is the strongest sign of a direct link between two texts. Agreement only in general shape, theme, or common phrasing points instead toward a shared tradition, since that is the kind of thing many writers in a culture would have held in common. Deciding which kind of agreement is present is the practical core of choosing among the three.
Why the Choice Matters
The three models are not just labels, because the same shared passage means something different under each one. If one text used the other, the useful question is how the later writer changed the earlier one and what the changes reveal about the writer's aims. If both used a shared source, the differences between the two show how each adapted material they held in common. If the resemblance comes from a shared tradition, then it is evidence about the shared stories of a whole culture rather than about one author reading another.
Getting the model wrong leads to one of two mistakes. The first is claiming that an author is alluding to a specific text he may never have read, when the likeness really comes from a shared tradition. The second is writing off close, word-for-word agreement as coincidence and missing a real literary link. Avoiding both means asking the questions in order, first whether there is any direct connection at all, and only then which text used which.