This paper is about Yaḥyā Ibn ‘Adī, a 10th century CE Christian philosopher and theologian, and his views on divine oneness (tawḥīd). Yaḥyā was a contemporary and peer of the foundational Islamic philosopher al-Fārābī (d. 950 CE), having studied alongside him under the tutelage of the scholarch of the Baghdad Peripatetics, Abu Bishr Mattā ibn Yūnus (d. 940 CE). Yaḥyā lived during the peak of the Graeco-Arabic translation movement during which Jews, Muslims, and Christians worked alongside one another with Abbasid state support to translate and study a wide range of Greek scientific, medical, and philosophical texts. In his Maqāla fī al-Tawḥīd (Treatise on Divine Oneness), Yaḥyā presents and refutes four ways that something may be called ‘one,’ before offering his own positive account of oneness as it applies to God, who for the Christian Yaḥyā is unique from one perspective and multiple from another. We focus on Yaḥya’s presentation and refutation of the second sense in which something which ‘has no equal’ (lā naẓīr lahu) is one. We argue that Yaḥyā here likely has al-Fārābī in mind, who describes in his Mabādi’ ahl al-Madīna al-Fāḍila (The Virtuous City) the absolute unity of the First Existent (al-mawjūd al-awwal). His existence has no equal; it shares nothing in common at all with the existence of any being. Examining Yaḥyā’s demonstrative proofs against the incoherence of such an understanding of divine oneness, we will evaluate Yaḥyā’s proofs and argue that they are sound and effective. They represent an early rationalist critique of al-Fārābī’s tendencies towards a theologia negativa. They also bear witness to the productive intellectual diversity of 10th century Abbasid Baghdad, showing that ‘Islamic’ philosophy includes important contributions by non-Muslims.

The central cosmogonic principle in Ibn Sīnā’s philosophy is the elegant rule that “from what is essentially one, only one thing may proceed.” Paradigmatically, this “Rule of One” describes the unique circumstances that regulate the atemporal ‘creation’ of the first creature, which Ibn Sīnā calls the First Intellect. It is only by the mediation of this single entity that God can be said to be the origin of multiplicity without compromising His transcendence and oneness. This theory became immediately controversial in the post-Avicennian phase of Islamic philosophy and theology. Modern studies on this issue tend to focus on the challenge the Rule of One posed on kalām creationism and divine voluntarism. However, some of the most compelling criticism of the Rule to emerge during this period seem to regard it not only as a cosmogonic principle; rather they hold Rule of One as a general causal principle that permeates the entirety of Ibn Sīnā’s cosmic system, including the lower strata of the physical world. Accordingly, their critique touched on broader issues related to metaphysics and natural sciences. One of the first thinkers to propose this interpretation is the Jewish philosopher Abū al-Barakāt al-Baghdadī (d. 1165). He initiated a line of inquiry that became highly influential during the period, especially for the Sunnī polymath and theologian Fakhr al-Dīn al-Rāzī (d. 1209). They argue that the Rule is operative in Ibn Sīnā’s account of the origination and inner structure of the human soul, i.e., in his theory of faculty differentiation, the soul’s governance of the body, and its transcendental origin in the Active Intellect. This paper will focus on this psychological dimension of the critique of the Rule of One and argue that it forms the starting point of Abū al-Barakāt and al-Rāzī’s reasoning for alternative models of the cosmic system, and ultimately of the doctrine of Divine Oneness itself.

Muslim discourses on unity (waḥdah) began with the rupture of the Quranic revelation in early seventh century Ḥijāz. Quranic hermeneutics anchored in unity of the Real (al-Ḥaqq) tend to subsume preceding and ensuing ontological, epistemological, and phenomenological distinctions and discourses. Earlier forms of such discourses include the Quranic chapter on sincerity (sūrah al-ikhlāṣ), the call to witness that there is no god but God (lā ‘ilāha illāllāh), traditions of Prophet Muhammad (d. 632) and philosophical sermons on unity and reality (ḥaqīqah) attributed to ʿAlī (d. 661). Subsequent Muslim encounter with Aristotelian writings brought into its fold the ideas of existence, substance, and essence; thereby extending the paradigmatic hermeneutics of unity and reality by entangling them with the problem of being. Instead of prioritizing the linguistic and poetic aspects of the Quranic discourse, philosophers like Fārābī (d. 950) valorized Aristotelian logic (manṭiq) and developed logical syntax of unity and existent (mawjūd) over and above their grammatical form in Arabic. Subsequently, Avicenna (d. 1038) transformed the debate between the Quranic notion of a thing (shay’) and an Arabic notion of existent (mawjūd) into a distinction between essence (māhiyyah) and existence (wujūd). By tying this distinction with the modal difference between the necessary and the contingent, he furnished a proof of God as the only necessary of existence in itself (wājib al-wujūd bī-dhātihi) while relegating unity to the level of one of the concomitants (lawāzim) of existence. On the other hand, many influential thinkers and poets including Rābiʿah (d. 801), Bāyazīd (d. 849), Ibn Ḥanbal (d. 855), Ḥallāj (d. 922), Niffarī (d. 965), ʿAyn al-Quḍāt (d. 1131), Sanā’ī (d. 1140), ʿAṭṭār (d. 1221), Ibn al-Fāriḍ (d. 1234) and Ibn ʿArabī (d. 1240) refrained from such overt rationalization. By appreciating the poetic, aesthetic, and imaginal aspects of reality and without relegating unity, they valued an untranslatable experience anchored in the sonorous effect of the Arabic revelation. Yet it is puzzling that those who claimed this legacy, in particular the commentators of Ibn ʿArābī, felt the need for developing a sophisticated rational framework, a theory, a discourse on the unity of existence (waḥdat al-wujūd) and proceeded to interpret the entire Quran and seminal poetic works from this perspective. In this paper, I will examine the philosophical implications for coming up with such a theory within the context of Quranic hermeneutics of unity and Avicenna’s notion of the necessary of existence.