Islamic jurisprudence, part of the larger system of law and ethics known as the Sharīʽa, recognizes four sources of law. The first two sources, the Qur’ān and the Sunnah, are its primary sources and must be consulted first by any jurist seeking to know what the Islamic law is on any particular topic. Although jurists often consult treatises reporting compilations of the fiqh (human understandings of the positive Islamic law) to ascertain what the law is, we should remember that the compilers of the books of fiqh produced that body of knowledge from examination of the Qur’ān and Sunnah. The Sunni jurisprudence and the Shi’a jurisprudence agree that the Qur’ān is the written and oral record of God’s speech to the Prophet Muhammad (s.a.w.s.), over a 23-year period beginning in 610 CE and concluding in 623 CE. It is the first source of the Sharīʽa in both systems of jurisprudence. Sunni and Shi’a jurists also do not disagree on the content of the Qur’ān. They may disagree on the interpretation of the words of the Qur’ān and the normative and legal judgments to be derived from readings or recitations of those words, but this is so even among the most learned of the Sunni jurists and exegetes. Sunni jurists and Shi’a jurists do disagree somewhat on the definition of the content of the Sunnah, although both systems of jurisprudence consider it to be a primary source of the law. Sunni jurists define it as the body of normative rules and life practice derived from a reliable record of the doings, sayings and tacit approvals of the Prophet Muhammad (s.a.w.s.), uttered and occurring while he was alive. The majority of the Shi’a jurists agree with this definition but also include the statements of imams they regard as infallible, beginning with Ali ibn Abi Talib, and they include those statements in the reliable record that is to be consulted when seeking to know the content of the Sunnah. This distinction, while extremely important, will not be of great concern to us because we will be focusing on ijmāʽ, the first of the two secondary sources of law.
The Sunni jurisprudence recognizes the two secondary sources of law to be ijmāʽ and qiyās. Shi’a jurisprudence also recognizes ijmāʽ as the first of two secondary sources but it takes quite a different approach toward the second of the two secondary sources, describing it as ‘aql or “rationality” or “intellect,” rather than employing the more narrow concept of qiyās, which is roughly translated as “reasoning by analogy.” Again, we will not be greatly concerned with this distinction as our focus will be on ijmāʽ, the third of the four sources.
Both the Sunni and Shi’a systems of jurisprudence largely agree on the definition and the juridical impact of an ijmāʽ, although we will see that there are also some methodological differences between the approaches of Sunni and Shi’a jurists in determining who decides the content of an ijmāʽ. These differences will likely not impede us in understanding what an ijmāʽ is and whether there can be an ijmāʽ on the question of the abolition of slavery.
The word ijmāʽ, literally meaning “determination” or “agreement” or “unanimity,” is a verbal noun derived from the Arabic verb ajmaʽa, which means “to determine” or “to agree upon something.” It can also mean “to collect” or “to unite, combine, bring together (parts into a whole).” The verb ajmaʽa has a much more comprehensive and wide-ranging set of meanings in Arabic but this brief reference to the basic meaning of the verb will suffice in helping us understand the meaning of ijmāʽ. Although the word ijmāʽ might be used in a variety of ways in everyday ordinary conversation, the term has taken on a specialized and technical meaning in the jurisprudence.
When used in jurisprudential discourse, the term is now usually translated as “consensus” and it describes “the unanimous agreement of the mujtahidūn [independent legal scholars] of the Muslim Community following the demise of the Prophet Muhammad on any matter.”
Some scholars hold that an ijmāʽ cannot be said to cover “any matter” but should be limited to covering only religious or juridical matters but, again, this will not concern us because slavery and its abolition are certainly matters that implicate religious and juridical decision making. The term, as used in this legal or jurisprudential sense, actually has pre-Islamic origins and was used in pre-Islamic Arabic to describe the consensus of opinion of members of a tribe on the validity or invalidity of a tribal practice or rule or custom. Its use in the language was closely related to references to the pre-Islamic Sunnah, which was the set of venerated precedents or traditions that shaped the expected behaviors of members of a particular tribe or clan or group.
The role of the pre-Islamic Sunnah changed greatly when the Qur’ān was revealed and both the Qur’ān and the Prophet Muhammad (s.a.w.s.) announced that Muslims should, as a matter of legal obligation, follow his Sunnah rather than the Sunnah of any particular tribe, clan, or group. As is well known, the Prophet Muhammad (s.a.w.s) united the peoples of the Arabian Peninsula and created a new juridical and religio-political entity, called the umma, which constituted all Muslims following the new religion and, by some accounts, even non-Muslims who were functioning rights-bearing members of the new community. The Sunnah of the Prophet became the dominant source of legal norms guiding this community, after the revelation of the Qur’ān, and the pre-Islamic Sunnah of the Arabian tribes became much less important as Islam took hold in the population, first on the Arabian Peninsula and then later across the Eurasian and African continents and in Southeast Asia. These historical events also similarly affected conceptions of the pre-Islamic ijmāʽ. The new conceptions of ijmāʽ were applied to events and issues experienced by the umma as a whole rather than events and issues that might be of concern to a particular tribe or group. In the post-prophetic jurisprudence that developed after the death of the Prophet Muhammad (s.a.w.s.), a close relationship emerged between the new Sunnah of the Prophet and these new conceptions of ijmāʽ. This relationship is most readily seen in the post-prophetic recognition of a concept known as the “ijmāʽ of the Companions,” best described as a unanimous consensus of opinion on a legal issue announced by the Companions of the Prophet Muhammad (s.a.w.s.) after his death. Their opinions on legal issue were considered to be authoritative because they knew, first hand, what the Prophet Muhammad’s (s.a.w.s.) vision for the new Islamic community or umma was and, if they unanimously agreed on a rule of law or norm or standard of behavior, their view became binding on the community, in the same way that the pre-Islamic ijmāʽ had been binding on a tribe or clan or group.
There are a number of famous examples of the “ijmāʽ of the Companions.” Perhaps the most famous is the process by which the community chose Abu Bakr to be the first Caliph after the Prophet Muhammad (s.a.w.s.)’s death. This was accomplished by a consensus of the Companions meeting at the saqifa (thatched-roof house) of the Banu Sa’ida clan in Medina. Abu Bakr’s subsequent decision to use military means to compel recalcitrant tribes to pay the zakat was also supported by the unanimous consensus of the Companions and is commonly cited as another early example of the use of consensus to support the validity of a juridical decision by the new Islamic state. There are a great many similar examples, especially from the time of the second Caliph, ‘Umar ibn al-Khattab. ‘Umar’s decisions with respect to the use and taxation of public land and the determination of the legal status of non-Muslim farmers and peasants on such lands were all made after extensive consultation with the Companions and the achievement of a unanimous agreement or ijmāʽ. Similarly, the establishment of the punishment for intoxication (80 lashes), the decision requiring husbands to pay full dowry after intimacy with a new wife, the adjustments made to Quranic inheritance rules to avoid unanticipated instances of unjust division of property, and the suspension of receipt of distribution of zakat to new Muslims in the community were all decisions reached during ‘Umar’s time via the implementation of an “ijmāʽ of the Companions.”
As the Islamic legal system matured and developed during the emergence and flowering of the Umayyad and Abbasid Caliphates, Sunni jurists functioning during those times began shaping the concept of “ijmāʽ into a mature and functioning legal doctrine. It was during this period that Sunni jurists agreed that there were only four sources of Islamic law and all of them included “ijmāʽ as one of those sources.
Many commentators point to al-Shaybani’s reliance on the prophetic ḥadith reciting the idea that “…Whatever the Muslims consider good is good in the sight of Allah, and whatever the Muslims regard as bad is bad in the sight of Allah” as the first appearance of a conception of ijmāʽ in the fiqh. Perhaps the most widely cited references to “ijmāʽ that were made in legal scholarship during this time were Imam Shafi’i’s discussions of “ijmāʽ in his two preeminent works of jurisprudence, al-Risāla and al-Umm. Shafi’i identified two kinds of consensus that were important for legal decision making, one being the consensus of the entire community and the other being the consensus of competent legal scholars. Although he argued vigorously for the predominant authority of the consensus of the community, using examples, it was the identification of the consensus of competent legal scholars that gained preeminence as a source of Islamic law in the Sunni legal discourse that followed the publication of Imam Shafi’i’s works. Although several prominent jurists after Imam Shafi’i, including Ahmed ibn Hanbal and Ibn Hazm, sought to limit the concept of ijmāʽ to the aforementioned “ijmāʽ of the Companions,” eschewing both consensus of the community and consensus of the scholars, this approach did not carry the day and the influential Sunni jurists who wrote during the medieval and early modern periods, particularly al-Ghazali, al-Razi, Ibn Qudama, al-Amidi, al-Mawardi, al-Sarakhsi, Ibn Taymiyya and others, all presented ijmāʽ, described as the consensus of competent legal scholars, as an important source of rule-making in Islamic jurisprudence.
There was a similar trajectory in the development of the concept of ijmāʽ among the Shi’a jurists. Shi’ism differs from Sunnism in that it concludes that Muslim communities should be led, from a political as well as from a religious and legal perspective, by a member of the ahl al-bayt (“people of the house”) that is, by a descendant of the Prophet Muhammad (s.a.w.s.). This conclusion is derived from interpretations of events that occurred during the life of the Prophet where it is believed by the Shi’a that he announced that the Muslim community should be led by a member of his family and that Ali ibn Abi Talib, his cousin and son-in-law, should be the first leader in this regard. There are now several schools of Shi’a jurisprudence that take different positions on various issues, but all agree that the Muslim community should be led by an infallible imam who is a descendant of the Prophet (s.a.w.s.), one who comes from the ahl al-bayt. All of the Shi’a communities also share a common belief with Sunnism in the primacy of the first two sources of Islamic law, the Qur’ān and the Sunnah.
With the exception of the Zaydi Shi’a, who continue to accept only the “ijmāʽ of the Companions,” all of the Shi’a communities use a conception of ijmāʽ in their understanding of what constitutes law, but their seems to be no agreement among them with respect to the authoritativeness of an ijmāʽ. The Twelver Shi’a community insists that the opinion of the Infallible Imam must be included in any determination of a consensus if it is to be considered valid and authoritative.  In some ways, this only means that Shi’a jurists use the doctrine of consensus as another way to ascertain the opinion of the Infallible Imam. If all competent Shi’a scholars agree on the point and they agree that one of the Infallible Imams held the same opinion, the determination is considered to be a proper ijmāʽ and Shi’a Muslims are bound to follow it as a binding rule of law.
A more involved discussion of the other Shi’a schools is currently being prepared to show that, by definition, a valid ijmāʽ must include the agreement of scholars of both Sunni and Shi’a Islam. Sources are unavailable due to libary closures resuting from COVID-19.
As we indicated, an ijmāʽ is the unanimous opinion of all the mujtahidūn of the Muslim community in any period of time on any matter, particularly on any juridical matter. This requires that at least a plurality of mujtahidūn, in both the Sunni and Shi’a communities, is currently in existence, or existed during a generational period in the past, and that such existence is ascertainable. There is no dispute that such a plurality currently exists, and has existed in the past, although there is some disagreement about who is qualified to be counted as a mujtahid(a). That determination is also one that is made by consensus, especially in the Sunni community. Imam Shafi’i laid down the basic qualifications of a mujtahid(a) and this was expanded upon and made complete by Abu’l-Husayn al-Basri. His statement of qualifications was later accepted, with minor changes, by Al-Shirazi, al-Ghazali, and al-Amidi, writing during medieval times. These statements have remained largely unchanged until today and are the subject of a consensus by all Sunni jurists. Generally, according to this view, to be qualified as a mujtahid(a), one must:
The mujtahid(a) must be of upright character and not a person who engages in transgressions. He or she must be free of conflict-of-interest and sincere. The Shi’a use a similar set of qualifications but they also use a more elaborate and hierarchical system of establishing such qualifications, requiring the candidate to demonstrate mastery of a number of religious sciences and other bodies of knowledge and to be vetted and approved by senior clerics in the Shi’a umma. Among the Shi’a the opinions of the mujtahid(a) are required to be obeyed by laypersons whereas among the Sunni, the layperson is free to reject the opinion of the mujtahid(a) and to apply their own informed judgment.
We emphasize that, according to a majority of the ‘ulama’, unanimity of the mujtahidūn of the Muslim community is a prerequisite of ijmāʽ. If there is a dissenting view, made in good faith by qualified mujtahidūn, this destroys the ijmāʽ. This is so even if the Sunnis are in unanimity and the dissenters are only from among the Shi’a jurists. A minority of the ‘ulama’ would also count the opinions of lay persons in determining whether there is an ijmāʽ, but this is a not a widespread view and is generally disregarded in deciding whether an ijmāʽ exists.
The presence of an ijmāʽ is generally ascertained by identification of an oral or written opinion, issued by a single competent mujtahid(a) or group of competent mujtahidūn, acting either individually or collectively as a group, or by identification of the opinions of a judge or group of judges, of which opinions the competent mujtahidūn become aware, and the mujtahidun either express their agreement with the original opinion, or opinions, of the judges, or they remain silent after having made aware of the opinion or opinions. The silence of the mujtahidūn is a particular kind of ijmāʽ, known as an ijmāʽ sukūtī (“tacit” or “silent” ijmāʽ) and proof of the awareness of the opinion or opinions by the mujtahidūn, followed by silence in circumstances where they would normally be expected to express their opinion, establishes that the ijmāʽ in fact exists. Such an ijmāʽ is just as authoritative and carries the same juridical weight in the jurisprudence as an express ijmāʽ.
The ijmāʽ cannot be the product of “pernicious innovation” or heresy or be a far-fetched or outlandish opinion. it must be the product of sincere, deliberate and thorough-going discourse and discussion by competent and upright mujtahidūn, all expressing their opinion or remaining silent during a particular age or period when the matter at issue is considered by jurists.
There has been much discussion on the authority of ijmāʽ, seeking to ground proof of its existence, and authority as a source of law, in the Qur’ān, the Sunnah, ijtihād, or other aspects of human reason.  Although all commentators have concluded that there is no decisive proof of the authority of ijmāʽ found in either the Qur’ān or the Sunnah, there is no doubt now, based on the great number of indications in those primary sources, and the reasoning of jurists interpreting those sources, that it is a valid and authoritative source of Islamic law and can be the basis for rule-making on juridical issues that are not definitively and conclusively resolved by the Qur’ān and Sunnah.
 Mohammad Hashim Kamali, Principles of Islamic Jurisprudence (Cambridge UK: Islamic Texts Society, 2003) (3rd rev. and enl. ed.), 16.
 Ibid., 229.
 Hans Wehr, A Dictionary of Modern Written Arabic (Arabic-English) (Ithaca, NY: Spoken Language Services, 1994) (1979) (4th ed.) (J. Milton Cowan, ed.).
 Kamali, Principles, 230. This is a paraphrase of a number of definitions, all closely hewing to this basic formula, and crafted in the classical period in the development of the jurisprudence.
 For an elucidation of what “any matter” actually covers, see Kamali, Principles, 230. An ijmāʿ cannot cover ideas touching on matters like the existence of God or matters that cannot be the subject of human legislation. Ibid.
 For discussion of the pre-Islamic meaning of ijmāʽ , see Wael B. Hallaq, Origins and Evolution of Islamic Law (Cambridge, UK: Cambridge University Press, 2005), 54.
 Wael B. Hallaq, Sharīʿa: theory, practice, transformations (Cambridge, UK: Cambridge University Press, 2009), 47-48.
 Hallaq, Origins and Evolution, 111.
 These examples are taken from Muhammad Y. Faruqi, “The Development of “Ijma’”: The Practices of the “Khulafa’ al Rashidun” and the Views of the Classical Fuqaha’”, American Journal of Islamic Social Sciences 9, No. 2 (Summer 1992): 173, 174-176.
 Ibid., 178, citing, at n. 35, Schacht and Hourani. See also Ahmad Hasan, The Doctrine of Ijma’ in Islam (Islamabad: Islamic Research Institute, 1978), 38.
 Faruqi, “The Development of Ijma’,” 178-182.
 Hallaq, Sharīʽa, 119-120.
 Kamali, Principles, 477.
 Ibid., 477-480.
 Hallaq, Sharīʿa, 113-124.
 Kamali, Principles, 234.
 The discourse on this topic is voluminous and extends back to the classical era. For recent commentary, and references to the classical discourse, see, e.g., Hasan, The Doctrine of Ijma’; Wael B. Hallaq, “On the Authoritativeness of Sunni Consensus,” International Journal of Middle East Studies 18 (1986): 427-454; George F. Hourani, “The basis of authority of consensus in Sunnite Islam,” in Hourani, Reason and Tradition in Islamic Ethics (New York and Cambridge, UK: Cambridge University Press, 1985) and Muhammad Qasim Zaman, “Rethinking Consensus,” in Zaman, Modern Islamic Thought in a Radical Age: Religious Authority and Internal Criticism (Cambridge, UK: Cambridge University Press, 2012), 45-74.
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