Agreement in principle that ijma binds has never settled agreement on what actually constitutes it, and the dispute has real consequences for how much of Islamic law counts as closed. Scholars differ over whether a valid ijma requires the agreement of every qualified jurist alive in a single generation, or something looser; over whether the agreement of the Prophet's Companions carries a decisive weight that a later generation's consensus does not; and over whether a consensus once formed can ever, in principle, be reopened by a later age that judges the earlier agreement to have rested on a mistaken premise. The Jafari school, the primary legal tradition of Twelver Shia Islam, accepts a version of ijma but grounds its authority differently from the Sunni schools: consensus is binding, on the Jafari account, not because scholarly agreement is inherently protected from error, but because the Imam is held to be present, even if unrecognised, among those who agree, so that it is ultimately his authority working through the consensus that makes it binding rather than the consensus as such. The Zahiri school, represented by the jurist Ibn Hazm, narrowed the question further still, accepting as binding only the consensus reached by the Prophet's own Companions and refusing the authority of any later scholarly agreement, a stricter standard than the other schools apply.