Al-Qāḍī al-Nuʿmān wrote a number of other treatises, several of which are not extant but whose titles show his engagement with Sunni Islamic legal theory and which may have been connected with his research and preparation for writing Ikhtilāf uṣūl al-madhāhib. His Kitāb al-ittifāq wa-l-iftirāq was presumably a work on the disputed points of the law (khilāf), including both Sunni and Ismaʿili law. He abridged this work as Kitāb al-muqtaṣar.
Al-Qāḍī al-Nuʿmān authored a number of polemical works related to law and legal theory. One of these, his Risālah dhāt al-bayān fī al-radd ʿalā Ibn Qutaybah (An Expository Treatise in Refutation of Ibn Qutaybah), is extant and has been published.14 This short work, written in response to a question posed to him by the tutor of the Fatimid princes, refutes legal doctrines discussed in the famous manual and anthology for secretaries Adab al-kātib by Ibn Qutaybah (d. 276/885). Avraham Hakim, the editor, dates the treatise to some point during the reign of al-Muʿizz after 343/954, because it mentions Ikhtilāf uṣūl al-madhāhib in one passage, and the Ikhtilāf dates to after 343/954. Two mentions in the treatise of the book al-Ikhtilāf wa-l-iftirāq suggest that al-Qāḍī al-Nuʿmān was planning to write the book while he completed the refutation of Ibn Qutaybah or that he was writing the two works simultaneously.15 Risālah dhāt al-bayān presumably predates Daʿāʾim al-Islām, which is not mentioned in the work.
Certainly related to the law and legal theory was al-Risālah al-miṣriyyah fī al-radd ʿalā al-Shāfiʿī (The Egyptian Treatise, A Refutation of al-Shāfiʿī), which reportedly consisted of two large fascicles. Poonawala suggests that this must have been composed shortly after or just before the Fatimid conquest of Egypt, apparently on account of the reference to Egypt in the title and the relative unimportance of Shāfiʿīs in Tunisia as opposed to their significant presence in Egypt. It appears likely, and this is suggested by the use of the term Risālah in the title, that this was a refutation of the theory of legal interpretation Muḥammad ibn Idrīs al-Shāfiʿī (d. 204/820) had presented in his Risālah.16 Al-Qāḍī al-Nuʿmān may have singled out al-Shāfiʿī on account of the leading role he played in the development of the science of legal hermeneutics. It is likely that this work included arguments similar to those found in Ikhtilāf uṣūl al-madhāhib, which at several points addresses arguments found in al-Shāfiʿī’s Risālah and in his other works.
Also likely devoted to matters of legal theory was al-Radd ʿalā Aḥmad Ibn Surayj al-Baghdādī, which critiqued the leading Shāfiʿī jurist of Baghdad in the late ninth and early tenth century, Aḥmad ibn ʿUmar Ibn Surayj (d. 306/918). This, too, was likely a refutation of a work on jurisprudence or legal theory rather than a work on positive law. It is likely that some of the opinions that are mentioned in Ikhtilāf uṣūl al-madhāhib but are attributed to unidentified authors or to “a certain Baghdadi,” are those of Ibn Surayj.
Al-Qāḍī al-Nuʿmān also wrote a refutation of a work by a Mālikī jurist whose name is not entirely clear in the title as reported in biographical and bibliographical sources: Dāmigh al-mūjaz fī al-radd ʿalā al-ʿUtbī or al-ʿUtaqī. If the first reading is correct, it refers to Muḥammad ibn Aḥmad al-ʿUtbī (d. 255/869), compiler of al-ʿUtbiyyah. If the second reading is correct, it is a refutation of the Mālikī scholar Ibn al-Qāsim (Abū ʿAbd Allāh ʿAbd al-Raḥmān al-ʿUtaqī, d. 191/806), a prominent disciple of Mālik ibn Anas and the compiler of al-Mudawwanah. The work was likely devoted to positive law.
Al-Qāḍī al-Nuʿmān’s last and most famous legal work is his Kitāb Daʿāʾim al-Islām. Poonawala argues convincingly that he wrote this work later than Ikhtilāf uṣūl al-madhāhib on the grounds that he had changed some of his views concerning legal theory between the composition of the two works. For the Daʿāʾim, he suggests a date of composition of 349/960, coinciding with a series of major reforms instituted by Caliph al-Muʿizz.17 This would confirm the supposition that he had completed Ikhtilāf uṣūl al-madhāhib some years earlier, in 343/954 or shortly thereafter. The Daʿāʾim is a two-volume work treating the standard chapters of Islamic law, prefaced by a section devoted to walāyah or “allegiance to the Imam.” It draws on al-Qāḍī al-Nuʿmān’s earlier legal works and presents a version of Islamic law that has been described as a compromise between Twelver and Zaydi Shiʿi law, rejecting, for example, the Twelvers’ position on mutʿah or temporary marriage and arguing against it on the basis of Zaydi sources. Daʿāʾim al-Islām has been published a number of times and translated into English as well. The recent translation by Ismail Poonawala has revised an earlier translation by Asaf A. A. Fyzee.18
Along with Kitāb al-Īḍāḥ, al-Qāḍī al-Nuʿmān’s massive collection of oral reports on legal topics, the two works Ikhtilāf uṣūl al-madhāhib, which propounds a theory of jurisprudence or legal hermeneutics, and Daʿāʾim al-islām, a compendium of rulings for the whole gamut of Islamic legal topics, provided the main basis for Ismaʿili law. Although the theories of religious authority and legal hermeneutics on which the law was based differed radically from those found in the other legal madhhabs of the time, these two works made Ismaʿili law conform formally to some of the main standards evident in other legal traditions. By the early tenth century, it had become necessary for all legal traditions to have a manual of uṣūl al-fiqh, or legal hermeneutics, in addition to recognized textbooks or compendia of the points of substantive law in order to be recognized as legitimate. Al-Qāḍī al-Nuʿmān single-handedly provided that basis, and his works remained the standard reference works for Ismaʿili law from that time until the present day.
It is probably on account of the fame of Daʿāʾim al-Islām that al-Qāḍī al-Nuʿmān has long been claimed by the Twelver Shiʿah as one of their own. Poonawala reports that the first Twelver biographical work to include him was Maʿālim al-ʿulamāʾ by Ibn Shahrāshūb (d. 588/1192). In that work, however, Ibn Shahrāshūb made it clear that al-Qāḍī al-Nuʿmān was not a Twelver: laysa bi-imāmī, though he immediately remarks “but his books are excellent.”19 Most subsequent Twelver biographical dictionaries not only include al-Qāḍī al-Nuʿmān but report or at least imply that he was a Twelver, including Majālis al-muʾminīn by al-Qāḍī Nūr Allāh al-Shushtarī (d. 1019/1610), Manhaj al-maqāl fī taḥqīq aḥwāl al-rijāl by Mīrzā Muḥammad al-Astarābādī (d. 1029/1619), Amal al-āmil by Muḥammad ibn al-Ḥasan al-Ḥurr al-ʿĀmilī (d. 1104/1692), Rijāl al-Sayyid Baḥr al-ʿUlum by Muḥammad al-Mahdī Baḥr al-ʿUlūm (d. 1212/1797), Rawḍāt al-jannāt by Muḥammad Bāqir al-Khwānsārī (d. 1313/1895), and Mustadrak al-wasāʾil by Mīrzā Ḥusayn al-Nūrī (d. 1330/1912).20 Muḥsin al-Amīn (d. 1952) also devoted a notice to al-Qāḍī al-Numʿān in his major biographical work Aʿyān al-shīʿah.21 The main source of this idea was the biographical dictionary Wafayāt al-aʿyān of Ibn Khallikān (d. 681/1282), who states that al-Qāḍī al-Nuʿmān was originally a Mālikī and then converted (taḥawwala) to Imami Shiʿism. Ibn Khallikān cannot have derived this information from Ibn Shahrāshūb’s Maʿālim al-ʿulamāʾ, which states explicitly that al-Qāḍī al-Nuʿmān was not an Imami, but it seems unlikely that he would have invented such a statement. Madelung and Poonawala have both noted that the eleventh-century Twelver Shiʿi scholar Abū al-Fatḥ al-Karājikī (d. 449/1057) wrote an abridgement of Daʿāʾim al-Islām and Sharḥ al-akhbār fī faḍāʾil al-aʾimmah al-aṭhār; this may be the source of the idea that al-Qāḍī al-Nuʿmān was actually a Twelver Shiʿi.22 Al-Qāḍī al-Nuʿmān’s arguments about the authority of the Imams and the historical wrongs they suffered at the hands of the Companions of the Prophet and later Sunni authorities clearly have resonated well with Twelver audiences and convinced many scholars over the centuries that he was indeed a Twelver. Any evidence to the contrary could be explained away as the result of dissimulation.