Thinkers like Corbin argue that the comparative study of philosophies must consider both similarities and differences. Initially, common elements are identified, followed by an exploration of divergences in relation to these shared features. Since all philosophical systems contain universal elements, they can be assessed within a hierarchical framework called “hierarchical comparison” (taṭbīq-i rotbī). This method is not limited to comparison; it also reveals deficiencies in each system and suggests ways to improve them internally.

The main criterion for ranking philosophical systems is their “most general conceptual principle,” typically their fundamental conception of existence. Its rank is determined by its “ontological scope,” the range of reality the system can theoretically comprehend. Other conceptual and propositional principles also influence the ranking. Hence, “extent of ontological coverage” becomes the key measure, with broader systems ranked higher. The relationship between higher and lower systems is analogous to that of Environment and enclosed.

Hierarchical comparison starts with selecting a specific issue, determining each system’s ontological scope, and establishing their rank, a stage called “external critique.” Comparison then proceeds in two ways: ascending or descending. In the ascending method, analysis begins with the lowest-ranked system. The issue is examined, and logical shortcomings are identified through “internal critique.” During or at the end of this process, the weakness of the system in correctly explaining the issue in question in some areas of reality becomes apparent, and it is shown how this weakness stems from some of the system's conceptual and confirmation principles. By modifying or replacing them, the system rises to a higher rank, repeating this process until the highest system is reached. In the descending method, first, the problem is explained at the highest-ranked system, after which corresponding elements are identified in lower-ranked systems for comparison.

Mulla Sadra’s treatment of the “three fundamental categories” (al-mavād ath-thulāth) exemplifies the ascending method. He ranks the systems as follows: the “personal unity of existence” (absolute divided existence) at the highest level, “specific gradation” (absolute existence with degrees) at the intermediate level, and the Peripatetic (Mashāʾī) system (concept of existence) at the lowest rank. Sadra begins using Peripatetic terminology but subjects it to internal critique, reconstructing the concepts of necessity, impossibility, and possibility. Necessity (wujūb) is defined as “existent by itself” (bayn bi-nafsihi), affirming existence and correcting logical inconsistencies in the Peripatetic framework.

External critique reveals the Peripatetic system’s ontological limitations due to its emphasis on conceptual spaces and essences (māhiyāt). Its threefold division fails to explain the actual existence of the Necessary Being. Sadra replaces these principles with the conceptual principle of “existential truth” and the propositional principle of “primacy of existence in the external world.” The Necessary Being is an entity whose existence derives from its essence, whereas possible beings require something else for realization. This shift from conceptual space to external reality forms the basis of Sadra’s reconstruction and the correct explanation of existence.

After analyzing the Peripatetic system and confirming specific gradation, Sadra establishes the “personal unity of existence.” Here, essences are realized through the unfolding of existential truth. Specific gradation acknowledges multiplicity across levels of existence, but personal unity of existence negates external multiplicity, treating all reality as a single being. Possible entities, attributes, and manifestations are aspects of this one being.

In the descending method, beings and their multiplicity are compared across the three systems. In the personal unity of existence, the primary principle is “absolute, undivided, unconditional existence,” which is limitless; reality consists of only one being. Multiplicity is explained via “aspect” (shan) and “essential connection” (‘ayn al-rabṭ), showing beings as relational manifestations of the single truth. Variations among beings reflect differences in intensity and weakness in “aspect-being,” arising from successive impositions of limitations.

In specific gradation, the principle is “absolute, unconditional existence with degrees.” Its propositional principle, the primacy of existence, affirms unity within multiple levels. Essences exist derivatively (mawjūd bi-tabi‘), realized according to their existential level. Multiplicity arises from variations in existential intensity and imposed limits, with the Necessary Being occupying the limitless highest level.

In the Peripatetic system, the conceptual principle is the “concept of existence” in the mind. Reality consists of multiple existents, with unity preserved only in conceptual understanding. Multiplicity is explained through general gradation (taškīk-i ‘āmī), reflecting differences in applying the general concept of existence to particular entities and distinctions in their essential limits.

Sadra’s hierarchical comparison demonstrates both the evaluation of systems according to their ontological breadth and their internal reconstruction, whether ascending from simpler frameworks or descending from the most comprehensive. Deficiencies are identified, principles refined, and a coherent system emerges that accounts for both unity and multiplicity, exemplified by Sadra’s personal unity of existence.