According to Twelvers, defining and interpretation of Islamic jurisprudence is the responsibility of Muhammad and the twelve Imams. As the 12th Imam is in occultation, it is the duty of clerics to refer to the Islamic literature such as the Quran and hadith and identify legal decisions within the confines of Islamic law to provide means to deal with current issues from an Islamic perspective. In other words, Twelver clerics provide Guardianship of the Islamic Jurisprudence, which was defined by Muhammad and his twelve successors. This process is known as "Ijtihad" and the clerics are known as "Marja'", meaning reference. The labels "Allamah" and "Ayatollah" are in use for Twelver clerics.