first four caliphs the Rashidun, meaning the "Rightly Guided", because they are believed to have followed the Qur'an and the sunnah (example) of Muhammad. Shi'a belief With the exception of Zaidis, Shi'ites believe in the Imamate, a principle by which rulers are imams who are divinely chosen, infallible and sinless and must come from the Ahl al-Bayt regardless of majority opinion, shura or election. They claim that before his death, Muhammad had given many indications, in the hadith of the pond of Khumm in particular, that he considered Ali, his cousin and son-in-law, as his successor. For the Twelvers, Ali and his eleven descendants, the Twelve Imams, are believed to have been considered, even before their birth, as the only valid Islamic rulers appointed and decreed by God. Shia Muslims believe that all the Muslim caliphs following Muhammad's death to be illegitimate due to their unjust rule and that Muslims have no obligation to follow them, as the only guidance that was left behind, as ordained in the hadith of the two weighty things, was the Islamic holy book, the Quran and Muhammad's family and offspring, who are believed to be infallible, therefore able to lead society and the Muslim community with complete justice and equity. The Prophet's own grandson, and third Shia imam, Hussain ibn Ali led an uprising against injustice and the oppressive rule of the Muslim caliph at the time at the Battle of Karbala. Shia Muslims emphasise that values of social justice, and speaking out against oppression and tyranny are not merely moral values, but values essential to a person's religiosity. After these Twelve Imams, the potential caliphs, had passed, and in the absence of the possibility of a government headed by their imams, some Twelvers believe it was necessary that a system of Shi'i Islamic government based on the Guardianship of the Islamic Jurist be developed, due to the need for some form of government, where an Islamic jurist or faqih rules Muslims, suffices. However, this idea, developed by the marja' Ayatollah Ruhollah Khomeini and established in Iran, is not universally accepted among the Shia. Ismailis believe in the Imamate principle mentioned above, but they need not be secular rulers as well. The Nizari continue to have a living imam; the current imam is the Aga Khan. The Taiyabi Ismaili have, since the year 1130, followed the imam's chief officer, the Dai al-Mutlaq, as they