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.