Considering the history of philosophy, one can see that the mental states of human beings are often explained based on the idea of the soul. The essence of the soul, though, has been a matter of dispute since the Greek philosophers, and various theories have been suggested. According to Plato, who considered the soul to be made up of an immaterial substance completely different from that of the body, it is capable of living without the body, has lived before it, and will continue to do so following its death. Aristotle, however, rejecting Plato‟s substantial dualist approach, defined the soul as the perfection of the body and, unlike Plato, denied its preexistence and immortality. Islamic philosophers, although accepting Aristotle‟s definition of the soul, regarded it as an immaterial substance and saw the human being as a composition of soul and body. Safavid period Islamic philosopher Sadraddin Shirazi put forward a new soul theory different from those of his predecessors. Like Ibn Sina and other Muslim philosophers, he accepted the Aristotelian definition of the soul as “the first actuality of a natural body equipped with organs”. However, to him, the organs in the definition are not material body parts but powers of the soul. Unlike Plato, he considered the soul originated. Nevertheless, as opposed to Ibn Sina who states that the soul comes into existence simultaneously with the body, Shirazi did not regard the soul as an immaterial being at the beginning of its existence. In his view, the soul comes into existence in the body. However, thanks to substantial motion, this material being evolves into an immaterial spiritual one. This article dealing with Sadraddin Shirazi‟s concept of the soul analyses his understanding of soul and spirit, the definition of the soul, its origination, and its relationship with the body.