The theory of jurisprudence that al-Qāḍī al-Nuʿmān proposes in Ikhtilāf uṣūl al-madhāhib has distinct Shiʿi elements but at the same time intensely engages with contemporary Sunni legal theory. One might describe this theory as recognizing three sources of law: the Qurʾan, the Sunnah, and the pronouncements of the rightful Imams.28 He assumes the authoritative status of the Book, as he terms the Qurʾan in this context in keeping with Sunni practice, and the Sunnah. He does not argue for their authoritative status, and while one might not expect this with regard to the Qurʾan because this was a matter of self-evident agreement and therefore not in need of such argument, one might expect it with regard to the Sunnah, for its exact role in the formation of the sacred law was hotly debated during the ninth and tenth centuries. Al-Shāfiʿi had famously justified the authority of the Sunnah in his Risālah by linking it with the term ḥikmah used in the Qurʾan in tandem with the term kitāb, and he also limited its definition to oral reports, and exclusively to oral reports concerning the Prophet.29 Al-Qāḍī al-Nuʿmān provides no such justification, which suggests that in his view, all Muslim jurists, or at least all whose opinions count, already agree on these two sources.
The question arises whether by Sunnah al-Qāḍī al-Nuʿmān intends reports that go back to the Prophet exclusively or whether he means to include reports that go back to the Imams as well. It is clear from his usage in Ikhtilāf uṣūl al-madhāhib that he intends by the term Sunnah the Practice of the Prophet as embodied in oral reports that go back to him. However, this may have resulted in part from the polemical nature of the work; he may be using the term as his opponents use it so as not to provoke an automatic rejection or argument on that particular issue. From the extant fragment of Kitāb al-Īḍāḥ it is evident that many of the akhbār or oral reports that are cited as evidence for particular legal positions are attributed to earlier Imams, especially Muḥammad al-Bāqir (d. 114/732) and Jaʿfar al-Ṣādiq (d. 148/765), and not to the Prophet. In the Ikhtilāf as well, reports going back to the early Imams are cited as evidence, though he does not use the term Sunnah to describe them. There is arguably some conflation of the two categories, on the understanding that the Imams are in many cases reporting material that has been passed down from the Prophet through their forefathers, his descendants. In al-Qāḍī al-Nuʿmān’s work as a whole, one would evidently draw the line between Sunnah and pronouncements of the Imams after Jaʿfar al-Ṣādiq: pronouncements from the Prophet and the Imams up through Jaʿfar al-Ṣādiq, which are available through Shiʿi compilations of law and ḥadīth, especially as compiled in Kitāb al-Īḍāḥ, and the pronouncements of the Fatimid caliph-imams, especially of the current Imam, which are available in other sources or directly from the Imam himself.
The pronouncements of the current Imam represent the third source, and al-Qāḍī al-Nuʿmān makes a significant effort to justify the Imams’ authority, this being understood as that of the Fatimid caliphs. His references to the current caliph, al-Muʿizz li-Dīn Allāh, as well as to the previous caliphs, and his pointed mention of the lifting of dissimulation (taqiyyah) in his time make this clear. In al-Qāḍī al-Nuʿmān’s view, this source is not an extraneous source divorced from the other two but is in fact intrinsic to the Qurʾan and the Sunnah. This is because both the Qurʾan and the Sunnah include unambiguous statements referring to the Imams’ authority. On the whole, this argument resembles al-Shāfiʿī’s argument for the authority of the Sunnah: It is not separate from the Qurʾan because the Qurʾan contains its explicit justification. The chief Qurʾanic prooftexts al-Qāḍī al-Nuʿmān cites are Q Nisāʾ 4:59 and Naḥl 16:43, both of which had become well-known authority verses among both Sunni and Shiʿi scholars by his time. The first includes the statement: aṭīʿū llāha wa-aṭīʿū r-rasūla wa-ulī l-amri minkum «Obey God and obey the Messenger and the Ones in Authority among you.» Al-Qāḍī al-Nuʿmān interprets ulū al-amr “The Ones in Authority” in this verse as an unequivocal reference to the Shiʿi Imams. The second verse includes the phrase, fa-sʾalū ahla dh-dhikri in kuntum lā taʿlamūn «So ask the people of knowledge if you do not know.» Again, al-Qāḍī al-Nuʿmān interprets the phrase ahl al-dhikr “the People of Knowledge” as an unambiguous reference to the Imams. He devotes some effort to justifying this view, arguing against various interpretations proposed by Sunni scholars that identify these phrases as references to the authority of military commanders, rulers, religious scholars, or jurists. Throughout the Ikhtilāf uṣūl al-madhāhib, al-Qāḍī al-Nuʿmān uses these two labels, “Those in Authority” and “the People of Knowledge” as technical terms referring to the Imams as the sole legitimate Islamic religious authorities. While he cites a number of other verses as justification for the Imams’ religious authority, these two are decidedly dominant in his discussion.30
Al-Qāḍī al-Nuʿmān also cites oral reports from the Prophet as justification for the Imams’ religious authority. Chief among these is ḥadīth al-thaqalayn “the Report of the Two Weighty Matters,” one of the chief oral reports cited in this fashion in Shiʿi tradition. This text mentions the Book—the Qurʾan—and ahl al-bayt “members of the Prophet’s family” as twin objects to which the believers must cleave after the demise of the Prophet in order to gain salvation. Again, for al-Qāḍī al-Nuʿmān, the term ahl al-bayt in the report is an unambiguous reference to the Imams. Therefore, just as the religious authority of the Imams is part and parcel of the Qurʾan, so too is it part and parcel of the Prophetic Sunnah.
With regard to oral reports, however, there are significant omissions. It is surprising, from the point of view of Sunni-Shiʿi polemics in this period, that al-Qāḍī al-Nuʿmān does not cite the report of Ghadīr Khumm or what is called ḥadīth al-manzilah “the Report of the Status,” two of the best known and most widely cited justifications of the authority of the Shiʿi Imams. According to the first report, the Prophet stopped at the pond of Ghadīr Khumm and addressed the Muslims who were returning with him to Medina after the Farewell Pilgrimage in the final year of his life. In his speech, he stated man kuntu mawlāhu fa-ʿAliyyun mawlāhu “Whoever I am the master of, ʿAlī is his master.” According to the Shiʿi interpretation, the Prophet intended by this the explicit designation of ʿAlī as his successor. In the second report, the Prophet is supposed to have said to ʿAlī that your position with regard to me is like that of Aaron with respect to Moses. Both reports were extremely influential for Twelver Shiʿah and for the Ismaʿilis in particular. They featured prominently in polemics between Sunnis and Shiʿah. On the basis of that first report, both the Fatimids and the Buwayhids instituted a Shiʿi holiday, ʿĪd al-Ghadīr, to be celebrated on the 18th of Dhu l-Hijjah, to mark the Prophet’s explicit designation (naṣṣ) of ʿAlī ibn Abī Ṭālib as his successor.
It is not clear why the report of Ghadīr Khumm does not appear in Ikhtilāf uṣūl al-madhāhib, but there must be a specific reason, since it is so well known and such a major justification of the Imams’ religious authority. Perhaps al-Qāḍī al-Nuʿmān saw that citing the Ghadīr Khumm report would lead directly into disputes about the Imamate in particular, which for this work would be a digression from the main topic. In contrast, in Daʿāʾim al-islām, he discusses Ghadīr Khumm in detail as a justification for allegiance