The relationship between mind and body has always been one of the most controversial issues in the history of philosophy. In Western philosophy, Descartes made a complete distinction between soul and body but could not explain the relationship between them because there were flaws in his theory at that time. Because of these problems, the school of parallelism emerged, and finally in the twentieth century, the philosophers of the mind rejected the incorporeity of the soul because it did not account for the relationship between the soul and the body. Moreover, physicalists, despite their denial of the soul and their attempts to explain the material relationship between mental states and the body, could not even provide a plausible explanation of this relationship and therefore this problem remained unresolved. However, according to the basic principles of his philosophy (the principality of existence and the gradation of being), Mulla Sadra was able to offer a plausible explanation of the relationship between soul and body according to which the soul and body are two aspects of a single existence and the body has been able to reach the stage of incorporeity through the substantial movement.