This dissertation examines the views of Fakhr al-Dīn al-Rāzī (d. 1210) as advanced in his two major philosophical works, al-Mabāḥith al-Mashriqiyya and al-Mulakhkhaṣ fī al-Ḥikma. It argues that Rāzī seeks to develop a philosophical programme that provides an alternative to the Aristotelian theory of scientific knowledge. The work is divided into two parts. Part I reconstructs the central components of Rāzī's logical system, including his theory of universals, his view of the role of definitions in philosophical analysis, and the alternative theory of predication that he advances in place of Aristotle's theory of predication. Part I focuses on the epistemological and logical programme that, in Rāzī's view, should precede the analysis of problems in the philosophical or post-logical part of the Mabāḥith and Mulakhkhaṣ (namely, Books I to III of both works). Part I consists of four chapters and a background discussion. The background discussion examines aspects of the Aristotelian theory of demonstrative science and Avicenna's interpretation of the Aristotelian theory, focusing on the nature of per se predication. Chapter 1 assesses the epistemological principles and views that Rāzī sets out in logic. Rāzī's discussion underscores a number of problematic epistemological assumptions in the Aristotelian theory of definition and concept acquisition, which he believes should not encroach on the logical analysis. Chapter 2 focuses on Rāzī's critique of per se predication on which demonstrative science is based and the alternative theory of predication that he advances. His alternative theory is based on the notion of "structured universals" as opposed to essences and per se properties. Chapter 3 examines Rāzī's critique of real definitions and assesses his view of nominal definitions. Rāzī advances nominal definitions as the alternative to real definitions. Chapter 4 examines how Rāzī's epistemological and logical programme informs his restructuring of philosophical discourse. I argue that the organization and order of the Mabāḥith and Mulakhkhaṣ are based on the alternative approach that he advances, which no longer preserves the standard ordering of the Aristotelian sciences. Here, metaphysics, construed as the highest science in the Aristotelian scientific system, no longer occupies a privileged position. Foundational ontological positions – such as, form-matter analysis, the theory of the four causes, or even atomism – are no longer presumed in the analysis of the nature of sensible objects, which Rāzī takes up in the lengthy Book II of the Mabāḥith and Mulakhkhaṣ. I conclude Part I with a postscript that examines aspects of the nature of Aristotelian logic, particularly in authors preceding Avicenna. Part II consists of two chapters, which examine his philosophical positions that follow, and are based on, his logical analysis, focusing primarily on views set out in Books I and II. Chapter 5 examines ontological problems relating to Avicenna's doctrine of the quiddity and Aristotelian form-matter analysis. It consists of a close textual analysis of a number of Rāzī's chapters in Book I of the Mabāḥith. I attempt to show that Rāzī read Avicenna's texts quite closely and that he sharply departs from Avicenna on central ontological questions. I argue that Rāzī's departure is informed by the philosophical programme that he advances in logic. Chapter 6 examines core elements of Rāzī's epistemology and psychology. The chapter expands on a number of epistemological problems that were only pointed out in his logical analysis, such as his rejection of the theory of mental forms. I argue that a core motivation for Rāzī's opposition to the Avicennan theory of mental forms derives from Rāzī's views on optics. Rāzī opposes the Avicennan theory of the "impression" of sensible forms (simulacra) and suggests that the perception of complex sensible forms involve processes that are more mind-dependent than allowed for by Avicenna's theory.