Monotheism constitutes one of the central doctrines of Islam. The notion is again and again voiced in the Qurʾān, thus for example in sūra 112 (entitled “Sincere Religion”) which, in the translation of Arthur Arberry, reads “Say: ‘He is God, One (ahadun). God, the Everlasting Refuge, who has not begotten, and has not been begotten, and equal to Him is not any one.” While initially it was apparently mostly a refutation of pre-Islamic polytheism in Arabia, the text was later on interpreted as primarily directed against the Christians.
The (post-Qurʾānic) Arabic term for monotheism is tawhīd. The frequent use of the root w-h-d in the self-appellation of numerous Islamic groups throughout the centuries up until the modern period indicates the central position the concept takes up in the self-perception of the Muslim believers. Mention should be made of the movement of the Almohads—“Almohads” being the Latinized rendering for al-Muwahhidūn, i.e., those who professed the unconditional unity of God (tawhīd)—a Berber dynasty that ruled a region extending from al-Andalus to Tunisia during most of the twelfth and part of the thirteenth century. The notion of tawhīd is also central to the doctrinal thought of Ibn ʿAbd al-Wahhāb (1703–92), a Hanbalī scholar from central Arabia whose theological vision was put into practice as a result of his allegiance with the central-Arabian amīr Muhammad b. Saʿūd, the founder of the Wahhābī-Saʿūdī state that eventually resulted in the modern state of Saudi Arabia, a country that has been instrumental in spreading the ideas of Ibn ʿAbd al-Wahhāb far beyond its borders. Taking his cue from the thirteenth century neo-Hanbalite theologian Ibn Taymiyya (1263–1328), Ibn ʿAbd al-Wahhāb drew a distinction between tawhīd al-rubūbiyya, the affirmation that God is the sole creator of the world, and tawhīd al-ulūhiyya or tawhīd al-ʿibāda, the notion that God is the sole object of worship according to the divine law. Another central feature of tawhīd according to Ibn ʿAbd al-Wahhāb is Islamic unity, and any kind of sectarianism or diversity is therefore to be rejected. During the twentieth century, Islamic activists increasingly singled out the notion of tawhīd to be the one defining doctrine of Islam, a development that was perhaps ushered in by the publication in 1897 of Muhammad ʿAbdūh’s (1849–1905) renowned Risālat al-Tawhīd. Considering tawhīd as the main organizing principle of human society, numerous activist organizations and Islamist parties adopted the term such as the “Dār al-Tawhīd” (“Abode of Unity”), a Shīʿī organization in the Gulf region, the Sunnī “Harakat al-Tawhīd” (“Unity Movement”) in Palestine, or the “Hizb-ut Tawhid” (“Party of Unity”) in Bangladesh.
But what does the notion of tawhīd, “monotheism” or “unity,” in fact stand for? The above-quoted Qurʾānic sūra conveys the notion of divine oneness, i.e., that God does not have a partner, no equal besides Him. This is also the understanding of the concept of tawhīd as expressed in the first half of the shahāda, the Islamic profession of faith developed during the post-Qurʾānic period, but is already implied in a series of Qurʾānic verses (2:255; 27:26; 28:70; 47:19, etc.). This shahāda, which constitutes the first of the so-called Pillars of Islam, is in fact the act of declaring “There is no god but God, and Muhammad is the Messenger of God.”
The renowned mystic Muhyī al-Dīn Ibn al-ʿArabī (1165–1240) laid the foundation for what became later the doctrine of the “unity of being” (wahdat al-wujūd) that proved influential ever since. Ibn al-ʿArabī distinguishes three levels of tawhīd: the absolute, undelimited and exclusive reality of the divine essence (al-ahadiyya al-ilāhiyya) that is devoid of any multiplicity as the highest level of tawhīd; inclusive unity (wahdāniyya/wāhidiyya) constituting the next layer in Ibn al-ʿArabī’s system that comprises the divine names and attributes, each one pointing to another aspect of the Divine. These are also the cause for the multiplicity of created beings, the loci in which God manifests Himself. The tawhīd al-dalīl finally constitutes the lowest level of unity in Ibn al-ʿArabī’s system and corresponds to the orthodox Islamic definition of tawhīd, i.e., the denial of polytheism as expressed in the Islamic profession of faith.
Among rational theologians, the mutakallimūn, it was primarily the question of the divine attributes and their ontological status and the manner in which they relate to the divine essence that was at stake. The Qurʾān asserts God’s omnipotence (“Indeed, God is over all things competent—innā Llāh ʿalā kull shayʾ qadīr,” as is stated in Q 2:20 and elsewhere) as well as His omniscience (“God is ever knowing and wise—wa-kāna Llāh ʿalīman hakīman,” Q 4:17 and elsewhere), as well as other attributes, and it states that God has “power” (qudra) and “knowledge” (ʿilm), etc. This gave rise to the controversial discussion whether “power,” “knowledge,” etc., constitute eternal attributes that are distinct from God’s essence or not. Assuming they were not, in what manner would His being powerful be distinct from His being knowing? Conversely, if they were distinct eternal attributes, they would constitute separate eternal ontological entities and, therefore, a plurality of eternal beings, rather than the one eternal God. Furthermore, with these eternal entities inhering in God, God himself would be compound, which implies plurality with respect to Him—a clear violation of the doctrine of divine unicity.
While traditionalist theologians considered any rational speculation over the dicta of the revealed sources to be impermissible and willingly accepted the evident contradiction between divine unity and a multiplicity of eternal attributes attached to the Divine by referring to the dogmatic injunction that the revealed sources need to be accepted “without asking how” (bi-lā kayfa), the issue took center stage among the rationalist theologians who were unwilling to compromise on the doctrine of tawhīd. The principal defenders of monotheism were the so-called Muʿtazila, the “People of Monotheism and Justice” (ahl al-tawhīd wa-l-ʿadl) as their adherents called themselves, a theological movement that flourished between the eighth and thirteenth century C.E.
As is the case with many aspects of Islamic religio-intellectual history, discursive theology in general and Muʿtazilite dialectical reasoning in particular were closely related in their evolution and development to parallel phenomena among the followers of other religions that were present in the Muslim world. The earliest preserved manifestations of discursive theology, “kalām” in Arabic, in Muslim circles can be traced back to the mid- or late eighth century. In two groundbreaking publications in 1980 and 1981, Michael Cook pointed out that characteristic features of Muslim kalām argumentation are already present in seventh-century Syriac Christological disputations and have some parallels in anti-Chalcedonian Syriac material as well. His findings have since been further refined.
The methodological tools of discursive theology had begun to leave their mark on Jewish thinkers writing in Arabic since the ninth century, and it seems that it was again the Christian kalām tradition that proved influential for the formation of Jewish medieval theology. The earliest extant Jewish kalām work is the ʿIshrūn maqāla, Twenty Chapters, of Dāwūd b. Marwān al-Muqammas, a student of the Syrian-Orthodox theologian Nonnus (Nānā) of Nisibis, who apparently flourished during the first half of the ninth century—so far the earliest theological summa in Arabic that we possess. As has aptly been shown by Sarah Stroumsa, it was primarily Nonnus’s characteristically Syriac Christian kalām—Aristotelian logic put to the service of Christian theology—that had “influenced and shaped al-Muqammas’s thought.” “Against the backdrop of the glaring absence of previous Jewish systematic philosophy” al-Muqammas “launched what was to develop into a remarkable tradition of Jewish rational thought,” to paraphrase Sarah Stroumsa’s evaluation of al-Muqammaṣ’s pioneering role in the evolution of a Jewish kalām tradition. The Kitāb al-Amānāt wa-l-iʿtiqādāt—that is, The Book of Beliefs and Opinions—of the tenth-century Rabbanite Jewish scholar Saʿadya Gaon (882–942) seems likewise to have been inspired by Christian theological literature as well as Islamic models. The Kitāb al-Tawhīd, The Book of Divine Unity, of Saʿadya’s Karaite contemporary Yaʿqūb al-Qirqisānī (d. 930) is unfortunately lost.
The new tradition of Jewish rational thought that arose during the ninth
century was in its initial stage primarily informed by Christian theological literature in content as well as methodology. Increasingly, specifically Muʿtazilite Islamic ideas, such as theodicy and human free will, as well as the stress on God’s oneness (tawhīd) resonated among Jewish thinkers, many of whom eventually adopted the entire doctrinal system of the Muʿtazila. The now emerging “Jewish Muʿtazila” dominated Jewish theological thinking for centuries to come.
The choice for Muʿtazilism was by no means self-evident. During the first half of the tenth century, a strong rival movement arose, named Ashʿariyya or Ashāʿira after its eponymous founder Abū l-Hasan al-Ashʿarī (d. 936), which soon gained in prominence. Following the Muʿtazilites methodologically, al-Ashʿarī—formerly a student of Abū ʿAlī al-Jubbāʾī, the leading figure of the Muʿtazila at the time—“converted” doctrinally to the theological views of the traditionists. In this he followed—and popularized—some of the views of the ninth-century theologian ʿAbd Allāh Ibn Kullāb (d. 855) who had already sought to amalgamate the discursive methodology of kalām with the traditional doctrinal notions of the traditionists in Arabic.
Unlike Muʿtazilism, Ashʿarism never really caught on among the Jews.
The famous Jewish thinker Moses Maimonides explains this Jewish predilection for Muʿtazilite kalām to be the result of mere chance: “… it has so happened,” Maimonides writes in the Guide of the Perplexed, “that Islam first began to take this road owing to a certain sect, namely, the Muʿtazila, from whom our coreligionists took over certain things walking upon the road the Muʿtazila had taken. After a certain time another sect arose in Islam, namely, the Ashʿariyya, among whom other opinions arose. You will not find any of these latter opinions among our coreligionists. This was not because they preferred the first opinion to the second, but because it so happened that they had taken over and adopted the first opinion and considered it a matter proven by demonstration.”
This explanation is certainly unsatisfactory. We may, however, gather some observations that may eventually help to explain this choice. The earliest attested Jewish compendium of Muʿtazilite thought is the Kitāb al-Niʿma, The Book of Blessing, of the Karaite Levi ben Yefet (in Arabic Abū Saʿīd Lāwī b. Hasan al-Basrī) (late tenth to early eleventh century), the son of the prominent Karaite Bible exegete and legal scholar Yefet ben Eli ha-Levi (whose Arabic name was Abū ʿAlī Hasan b. ʿAlī al-Lāwī al-Basrī) (d. after 1006). Levi wrote the book at the request of his father as a vindication of Judaism on the basis of Muʿtazilite rational theology, but unlike his father, who disapproved of Islamic Muʿtazilite theology, Levi adopted the doctrines of the Muʿtazila and implicitly recognized Muhammad as a friend of God endowed with prophethood, though ranking below Moses. Further evidence as to when (and why) Jewish thinkers began to adopt Muʿtazilite thinking can be gleaned from the extant Jewish copies of Muʿtazilite works of Muslim representatives of the movement, as preserved in the various Genizah collections, most specifically the Abraham Firkovich collection in St. Petersburg. Although a full inventory of the relevant collections and its Muʿtazilite materials is still a major desideratum, it seems that the writings of the Būyid vizier and patron of the Muʿtazila, al-Sāhib b. ʿAbbād (938–95), who was himself an adherent of the movement, constitute the earliest Muslim Muʿtazilite works, copies of which can be traced in the various Jewish collections. Moreover, it is attested that Jewish theologians regularly participated in the majālis convened by Ibn ʿAbbād at his court in Rayy, the most important center of Basran Muʿtazilism during the vizierate of Ibn ʿAbbād (976–95), although we do not possess any names of the Jewish theologians who flourished there.
While these observations do not shed any light as to why Jewish thinkers started to adopt Muʿtazilite doctrines, they suggest, however, that the major turn toward Muʿtazilism occurred during the later decades of the tenth century, i.e., only some few decades after the lifetime of Saʿadya Gaon. Levi ben Yefet’s summa was soon eclipsed by the theological writings of the Rabbanite Samuel ben Hofni Gaon (d. 1013) and his Karaite opponent and younger contemporary Abū Yaʿqūb Yūsuf al-Basīr (d. between 1037 and 1039), whose kalām works gained an almost canonical status among the Karaites. Literary evidence suggests that Muʿtazilite ideas constituted the central doctrinal foundation of the Rabbanite community until the middle of the twelfth century. For the Karaites, Muʿtazilism continued to provide a significant doctrinal framework at least through the seventeenth century, an observation that also applies to the Byzantine Karaite milieu where many of the works originally composed in Arabic were transmitted in a Hebrew translation.
The most important center of Jewish Muʿtazilism during those centuries was Baghdad, which was soon replaced by Jerusalem and, following the Crusaders’ capture of Jerusalem in 1099, Old Cairo (Fustāt).
The emergence and historical development of the “Jewish Muʿtazila” is not only an interesting phenomenon in itself—its literary testimonies also fill a glaring gap in the primary sources for the Muslim Muʿtazila that are available to modern scholarship. During the vizierate of Ibn ʿAbbād, Rayy was the unrivalled center of Muʿtazilism. It was here that ʿAbd al-Jabbār al-Hamadānī (ca. 937–1024) was appointed chief judge in 977, a position he held until the death of his patron Ibn ʿAbbād in 995. In his function as head of the Muʿtazilite school of the Bahshamiyya, ʿAbd al-Jabbār assembled a large circle of students around him. Ibn ʿAbbād in turn initiated the foundation of a library that is said to have held between 100,000 and 200,000 volumes, making it one of the largest collections of books in the Islamic world at the time. When in 1029 Mahmūd Ghaznawī entered Rayy, the library was partially destroyed, including its Muʿtazilite holdings, and many adherents of the movement were driven out of the city. Muʿtazilism only survived within the Zaydī circles of Northern Iran, specifically Rayy and Bayhaq. Following the unification of the Zaydī state in Northern Iran with their coreligionists in Yemen during the thirteenth century, a massive transfer of Zaydī and non-Zaydī religious literature from Iran to Yemen occurred that also included a large amount of Muʿtazilite literature. However, the Zaydīs only preserved a specific layer of Muʿtazilite writings, most of which consists of the works of Zaydī and non-Zaydī students of ʿAbd al-Jabbār. They did not preserve any of the writings of ʿAbd al-Jabbār’s predecessors, and even of ʿAbd al-Jabbār himself, they only had his comprehensive al-Mughnī fī abwāb al-tawhīd wa-l-ʿadl, The Sufficient [Book] on the Matters of Unity and Justice, at their disposal. Other works of his were either not transmitted or preserved as paraphrastic renderings (for example his al-Kitāb al-Muhīt, which only came down to the Zaydīs of Yemen as the al-Majmūʿ fī l-muhīy of Ibn Mattawayh).
By contrast, the Jewish Muʿtazilites preserved an earlier layer of Baṣran Muʿtazilite literature, namely numerous writings of ʿAbd al-Jabbār, many of which are otherwise only known by title, including commentaries by ʿAbd al-Jabbār on a work by Abū Hāshim al-Jubbāʾī on natural philosophy and on a theological text by Ibn ʿAbbād. In addition to this, extensive fragments of what seems to have been a voluminous theological summa by Ibn ʿAbbād have been preserved, as well as a work on natural philosophy by ʿAbd Allāh b. Saʿīd al-Labbād, another prominent student of ʿAbd al-Jabbār whose works soon fell into oblivion among the Zaydī Muʿtazilites.
By way of illustration, I shall briefly refer to the case of Ibn ʿAbbād’s theological summa, possibly his Kitāb Nahj al-sabīl fī l-usūl al-dīn, The Book of the Procedure Along the Way on the Principles [of Religion]. Islamic historical sources inform us that Ibn ʿAbbād had composed comprehensive theological works, but none of these has been preserved in the Islamic world. So far we only possess some concise theological tracts of his that appear to have been written as introductions to the doctrine of the school. That he was widely read within Jewish Muʿtazilite circles is evident from two extensive fragments of a theological summa of his that are both written in Hebrew characters. Unlike the concise tracts that are preserved in Islamic collections, these fragments (which are now available in critical edition) clearly show that al-Sāhib was not only an adherent of the Muʿtazila but a theologian in his own right. Moreover, as I suggested before, his writings may have played a decisive role in the formation of the Jewish Muʿtazila.
This example—one out of many—also illustrates what students of Muslim intellectual history can gain by looking for relevant source material beyond strict denominational borders. The scholarly investigation of the Jewish Muʿtazila, its historical connection to Muslim counterparts, and a systematic exploitation of the Islamic primary materials preserved in Jewish collections, are still in their infancy. While representatives of the “Wissenschaft des Judentums” (“Science of Judaism”) toward the end of the nineteenth and beginning of the twentieth century, such as David Kaufmann, Martin Schreiner, or Arthur Biram, were aware of this important episode, the rise of the Nazi regime in Germany and World War II put an end to this early attempt to study Muslim and Jewish Muʿtazilites as part and parcel of one single intellectual phenomenon and to analyze the historical relations between them. It was only later that scholars of both Jewish and Islamic studies “rediscovered” this important field and joined forces to work on the relevant materials.
An earlier version of this paper was presented at the Holberg Prize Symposium: Ancient Religions, Modern Dissent, Bergen, June 2014. Sabine Schmidtke, Professor in the School of Historical Studies, has played a central role in the exploration of heretofore unedited and unknown theological and philosophical writings. Schmidtke has applied rigorous study to the edition and critical analysis of manuscripts in Arabic, Judeo-Arabic, and Persian, and her work extends from Arabic-speaking countries to Israel, Iran, Russia, and Turkey.
 See Alexander Treiger, “Origins of Kalām,” The Oxford Handbook of Islamic Theology, ed. Sabine Schmidtke (Oxford: Oxford University Press, 2016): 27–43
 The work has recently been reedited. See Dāwūd ibn Marwān al-Muqammas’s Twenty Chapters (ʿIshrūn maqāla), an edition of the Judeo-Arabic text, transliterated into Arabic characters, with a parallel English translation, notes, and introduction by Sarah Stroumsa (Provo, Utah: Brigham Young University Press, 2016).
 Moses Maimonides, The Guide of the Perplexed, trans. Shlomo Pines (Chicago: The University of Chicago Press, 1969): 176f.
 Al-Sāhib Ibn ʿAbbād Promoter of Rational Theology: Two Muʿtazilī kalām texts from the Cairo Geniza, ed. Wilferd Madelung and Sabine Schmidtke (Leiden: Brill, 2016).