In New Testament studies, source criticism has taken a slightly different approach from Old Testament studies by focusing on identifying the common sources of multiple texts instead of looking for the multiple sources of a single set of texts. This has revealed that the Gospels are both products of sources and sources themselves. As sources, Matthew, Mark and Luke are partially dependent on each other and partially independent of each other. This is called the synoptic problem, and explaining it is the single greatest dilemma of New Testament source criticism. Any explanation offered must "account for (a) what is common to all the Gospels; (b) what is common to any two of them; (c) what is peculiar to each". Multiple theories exist to address the dilemma, with none universally agreed upon, but two theories have become predominant: the two-source hypothesis and the four-source hypothesis.