Rida differentiated between three types of caliphate. First, there was the ideal caliphate, which existed under the Rashidun and the pious Umayyad Caliph Umar Ibn Abd al-Aziz, "the fifth of the Rashiduin." The caliph had in this case all the shar'i requirements. The second type of caliphate was "the caliphate or imamate of necessity." This type would be allowed when the "ahl al-hall wa'l 'aqd" decided to install a caliph who had most but not all the legal requirements. To this type of caliphate belonged some of the Umayyad and Abbasid caliphs who lacked 'ilm (knowledge), imperative in exercising ijtihad. The third type of caliphate was the caliphate of tyranny or conquest ("al-taghallub bi'l quwwa"), which was contracted by force without the "ahl al-hall wa'l 'aqd". For Rida, it denoted that worldly leadership had become wholly dependent on "shawka", or force. To this last type belonged the Ottoman caliphate, according to Rida at this point. Although he said that such a caliphate should be obeyed because "necessity makes lawful what is otherwise forbidden,"' Rida also mentioned that it should be overthrown whenever it became possible to do so.