Muslim philosophers preceding Mulla Sadra unanimously believed that the soul, like immortal intellect, is in essence free from matter. The difference, however, is that unlike intellects, the soul is in need of a body to perform its actions. These actions, carried out under the auspices of the soul, give rise to the development of the soul. Accordingly, they held that the definitions presented for the soul were drawing on I-ness, that is, its dependence on the body and as a result they do not show the reality of the soul.But in the light of the principles of his transcendental theosophy - the fundamentality of being, the gradational nature of being and the substantial motion of the soul - Mulla Sadra offers a new perspective which has become the basis for his philosophical psychology. Refuting the idea of the previous philosophers who believed that the relation of the immaterial soul to the material body is something extrinsic and beyond the essence of the soul, he argues that the interdependence of the soul and the body is intrinsic to the soul itself. Stated differently, the soul exists in a way that it pertains to the body.To substantiate his claim, Mulla Sadra firstly argues that if the soul was essentially free from matter, as is generally believed, it would be impossible for it to be related to the material body. Secondly, if the soul was immaterial intellect, it would be impossible that a single natural species called ‘man’ comes out of the synthesis of an immaterial substance and a material body.Based on the essentiality of the dependence of the soul on the body, Mulla Sadra resolves the rest of issues in philosophical psychology. By considering the temporal soul as being material, he offers various expositions on the relationship between the soul and body and the cause of natural death. He further formulates new arguments to disprove the doctrine of metempsychosis.