If this document existed, it has now been lost, but some of its material can be deduced indirectly. There are five highly detailed arguments in favor of Q's existence: the verbal agreement of Mark and Luke, the order of the parables, the doublets, a discrepancy in the priorities of each gospel, and each one's internal coherence. Q allowed the two-source hypothesis to emerge as the best supported of the various synoptic solutions. There is also material unique to each gospel. This indicates additional separate sources for Matthew and for Luke. Biblical scholar B. H. Streeter used this insight to refine and expand the two-source theory into a four-source theory in 1925.