It is doubtful whether Mithraism had a monolithic and internally consistent doctrine. It may have varied from location to location. However, the iconography is relatively coherent. It had no predominant sanctuary or cultic centre; and, although each mithraeum had its own officers and functionaries, there was no central supervisory authority. In some mithraea, such as that at Dura Europos, wall paintings depict prophets carrying scrolls, but no named Mithraic sages are known, nor does any reference give the title of any Mithraic scripture or teaching. It is known that initiates could transfer with their grades from one Mithraeum to another.