Existing studies have predominantly conceptualized the development of schools of Islamic philosophy in terms of methodological differences and their respective approaches to philosophical questions, without analyzing this development through the lens of the relationship between “knowing” and “being.” Seeking to address this gap, this article, employing an analytical-comparative method, first develops a three-level framework for conceptualizing philosophizing and then uses this framework to reinterpret the development of Peripatetic philosophy, Illuminationist philosophy, and Transcendent Philosophy. The findings indicate that in Peripatetic philosophy, insofar as knowledge is conceived as a quality of the soul, no structural connection is established between knowledge and the existential transformation of the soul. In Illuminationist philosophy, despite the connection between knowledge, intuition, and the purification of the soul, the relationship between the act of intellection and the existential intensification of the soul remains insufficiently explained. By contrast, drawing on three principles—the substantial and intensifying motion of the soul, an existential conception of knowledge, and the theory of the identity of the knower and the known—Mulla Sadra is able to formulate philosophical knowledge and the existential transformation of the knower within a unified framework. Accordingly, philosophizing in Transcendent Philosophy is not merely a mental activity aimed at acquiring true propositions; rather, it is an existential-rational process that constitutes the very ontological movement of the soul.