The point to be emphasized is that, in the present state of opinion upon the Synoptic problem, the difference of one Evangelist from another does not in itself invalidate the testimony of either. The Synoptic problem, while primarily a literary problem, is indeed "fraught with momentous issues which the Church, and not scientific criticism only, is concerned to face";[287] but in the present state of the discussion, the fact that Matthew adds to or modifies the narrative of Mark does not necessarily place the Matthean modification upon a lower plane of credibility than the Markan statement. The Matthean modification may be an exact copy of an earlier edition of Mark, or may be derived from one of Mark's sources, Q, or may be taken from that stream of oral tradition coming from "eye-witnesses and ministers of the word," which Luke in his preface evidently regarded as the touchstone of historical truth, whatever his use of written sources. 287: H. L. Jackson, in "Cambridge Biblical Essays," 1909, p. 432. Passing over the vexed question of Q, we may observe that the acceptance of Harnack's early dating of the Acts and Luke would further complicate the two-document theory. He agrees that Luke was written before the Acts, and the Acts before Paul's trial at Rome was decided; further that Mark is one of the sources of Luke, and that Mark was written at Rome. "Tradition asserts no veto against the hypothesis that Luke, when he met Mark in the company of Paul the prisoner, was permitted by him to peruse a written record of the Gospel history which was essentially identical with the Gospel of Mark given to the Church at a later time." Perhaps, he intimates, "Luke was not yet acquainted with Mark's final revision, which, as we can quite well imagine, Mark undertook while in Rome."[288] The priority of Mark, under this supposition, is left hanging by a slender thread. It is highly probable that Luke gathered the material for his work (and a great part of it was certainly independent of Mark) while in Palestine, and if he did not see Mark's Gospel, or a rough draft of it, until he was in Rome, it is improbable that the Markan document was his primary and principal source, as the two-document theory asserts. 288: IV, p. 93; "Date of the Acts and of the Synoptic Gospels," p. 133.