The "Twelve Imams" are the spiritual and political successors to Muhammad for the Twelvers. According to the theology of Twelvers, the successor of Muhammad is an infallible human individual who not only rules over the community with justice but also is able to keep and interpret the divine law and its esoteric meaning. The words and deeds of Muhammad and the imams are a guide and model for the community to follow; as a result, they must be free from error and sin, and Imams must be chosen by divine decree, or "nass", through Muhammad. Each imam was the son of the previous imam, with the exception of Hussein ibn Ali, who was the brother of Hasan ibn Ali. The twelfth and final imam is Muhammad al-Mahdi, who is believed by Twelvers to be currently alive and in occultation.