In his Four Gospels, Burnett Streeter articulates his view of the sources of Luke and proto-Luke as follows:
The hypothesis I propose in no way conflicts with the generally accepted view that Matthew and Luke are ultimately dependent not only on Mark but on Q—meaning by Q a single written source. Most, if not all, of the agreements of Matthew and Luke, where Mark is absent, are, I think, to be referred to Q; but I desire to interpolate a stage between Q and the editor of the Third Gospel. I conceive that what this editor had before him was, not Q in its original form—which, I hold, included hardly any narrative and no account of the Passion—but Q+L, that is, Q embodied in a larger document, a kind of “Gospel” in fact, which I will call Proto-Luke. This Proto-Luke would have been slightly longer than Mark, and about one-third of its total contents consisted of materials derived from Q (Streeter 208).
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