This study examines how Ibn Ḥazm (d. 456 AH/1064 CE) articulated his nominalist critique of Platonic realism in the context of a larger rejection of ontological dualism in philosophy. It draws on evidence in Al-Fiṣal fī l-Milal wa-l-Ahwāʿ wa-l-Niḥal (The Book of Opinions on Religions, Heresies, and Sects) and his Marātib al-ʿUlūm (Categories of the Sciences). In response to those who “claim to follow philosophy (falsafa),” and in dialogue with earlier theologians and philosophers such as al-Bāqillānī (d. 403/1012–1013) and al-Kindī (d. 258/873), Ibn Ḥazm redefined the universal soul (al-nafs al-kulliyya) and universal intellect (al-ʿaql al-kullī) as linguistic references to the total of all particular souls and particular intellects, which he defined as corporeal accidents inhering in the body. Ibn Ḥazm’s identification of souls and intellects as corporeal was part of his larger conception of the world as discrete and finite in both space and time. The world, in other words, is measurable in numbers and therefore limited by the volume of its visible and invisible air-like corporeality to the exclusion of philosophical notions of a perfect void or prime matter. In his additional critique of contemporary Muslim epistemology and the theologians’ reliance on dialectical argumentation, Ibn Ḥazm held that a true scholar of Islam should turn to logic-oriented deductive methods and scriptural evidence together in order to ascertain the possibilities and, more importantly, the limits of human knowledge about both the corporeal created world and the ontological unknown (ghayb) of the divine realm.