While most scholars agree that the two-source theory offers the best explanation for the Synoptic problem, and some say it has been solved, others say it is not solved satisfactorily. Donald Guthrie says no single theory offers a complete solution as there are complex and important difficulties that create challenges to every theory. One example is Basil Christopher Butler's challenge to the legitimacy of two-source theory, arguing it contains a "Lachmann fallacy" that says the two-source theory loses cohesion when it is acknowledged that no source can be established for Mark. F. C. Grant posits multiple sources for the Gospels.