Due to shortcomings in Descartes’ premises and arguments, the immateriality of the soul is not rigorously proven in his work; consequently, this has even led to the rejection of the very concept of the soul’s immateriality within Western philosophy. The objective of this paper is to identify—based on Sadrian philosophy—which of Descartes’ explanations regarding the immateriality of the soul have led to his inability to justify that immateriality. This study employs a qualitative approach and a descriptive-analytical method. This study has identified, based on Sadrian perspectives, eight instances of Descartes' shortcomings in justifying and proving the immateriality of the soul. The findings of this study can be categorized under the following four general axes: 1. inability to account for the differences among mental states and his categorization of them all under the heading of "thought." 2. inability to explain how a real union between soul and body is possible. 3. entrapment in the contradiction between the claim of the existential independence of the soul and the body and the claim of their real union and 4. unable to explain why bodily changes exert profound effects on mental states.