Ijma is the verbal noun of the Arabic word Ajma’a which has two meanings : to determine, to agree upon something. Ijma is considered the third proof of Shariah after the Quran and the Sunnah. As a proof of Shariah, it is basically a rational proof. An Ijtihad or an Interpretation of one or a few scholars when becomes universal, becomes Ijma.
The classical definition of Ijma, as laid down by Ulama of Usul, is categorical on the point that the universal consensus of the scholars of the Muslim community as a whole can be regarded as conclusive Ijma. Only such Ijma are considered binding by early Usuliun (Usul scholars). However universal Ijma are indeed very few. As evidence show, it is extremely difficult to prove Ijma on particular issues, particularly in the case of issues open to ijtihad or tawil. There is no authentication of Ijma through Isnad (chains of narrators).
The only form of Ijma upheld by majority is the Ijma of Sahabis only. Majority of Ulama of Usul think that Ijma can take place on Sharii and devotional (Ibadah) and dogmatic (Itiqad) matters. For the first time Ijma occurred among the companions of the Prophet (SM). Ijma initially helped unity of Ummah in some matters. Ijma also ensures correct interpretation as broad consensus is unlikely to take place on incorrect matter. Ijma also enhances the authority of the rule on which there is Ijma. Unanimity of Ulama on an issue of a particular time is a requirement of Ijma. The agreement must be expressed by clear opinion of all scholars of the time. Ijma must consist of the agreement of all majtahidun. Though many Ulama consider majority to consist Ijma.
Any agreement of majority can be a proof but can not be a binding proof because to be binding, it must fulfill the conditions stated in the Ahadith quoted in support of Ijma (which is nothing short of Ijma of all people, at least all scholars.) There is no good ground to exclude any scholar of any school of Islam, as long as the school or group itself is not considered outside Islam by the Muslims.
The Ulama have on the whole maintained that the textual evidence in support of Ijma does not amount to conclusive proof. The Ayats quoted in support of Ijma (4:59, 4:83; 4:115, etc.) are not conclusive for Ijma. Imam Gazali says these Ayats are indications, not clear Nass on Ijma. Suyuti’s interpretation is the same. Abduh does not find any Ijma in these Ayat. Al Amidi says, “these give rise to probability (Zann), not positive knowledge”. (see Kamali).
About 10 Ahadith are quoted in support of Ijma (see Kamali). Ahmed Hassan observes that these hadith are inconclusive on Ijma. A number of Ulama (including Shafii and Mutazila scholars) have said that Ijma of classical definition is not feasible because of the huge number of the Ummah or its scholars or distances. It is for this reason that Imam Shafii confines the occurrence of Ijma to the obligatory duties only. For the same reason, Zahiris and Imam Ahmad refer by Ijma to the consensus of companions only.
Abdul Wahab Khallaf is of the view that Ijma of classical definition is no longer possible in modern times (because of huge number of scholars spread over continents). Khallaf is right. Old style Ijma is no longer possible. You can have only local Ijma, which is useful in lawmaking through Parliament but they can not be (by definition) binding forever.
Ijma are of two types – Ijma al Sariah (explicit Ijma) and Ijma al Sukuti (Ijma by silence). Ijma Sukuti (which occurs when one or a few scholars agree on something and no dissent is known) is not a proof according to a majority of scholars. According to the majority Ijma must be founded in a Textual authority (Quran and Sunnah) or Ijtihad. There are 3 views on whether Qiyas can be a basis of Ijma or not (see Kamali). Some agree, some disagree, some partially agree.
Ijma can be transmitted by Ahad or Mutawatir report of scholars. There is no Mutawatir report of Ijma except those of Ijma of companions. Iqbal gives a proposal to transfer performance of Ijma to the legislative assembly, which is only possible form of Ijma in modern times. Iqbal is right. His ideas require acceptance. However, such Ijma can not be of universal validity nor can it be considered binding (unless made into a local law – which then remains valid until revoked). In conclusion we can say that Ijma can be of limited use only in future. Qiyas, Istihsan, Maslaha are more important in future.