The discussion of the human soul, its existence, nature, ultimate objective and eternity, occupies a highly important position in Islamic philosophy and forms its main focus. For the most part Muslim philosophers agreed, as did their Greek predecessors, that the soul consists of non-rational and rational parts. The non-rational part they divided into the plant and animal souls, the rational part into the practical and the theoretical intellects. All believed that the non-rational part is linked essentially to the body, but some considered the rational part as separate from the body by nature and others that all the parts of the soul are by nature material. The philosophers agreed that, while the soul is in the body, its non-rational part is to manage the body, its practical intellect is to manage worldly affairs, including those of the body, and its theoretical intellect is to know the eternal aspects of the universe. They thought that the ultimate end or happiness of the soul depends on its ability to separate itself from the demands of the body and to focus on grasping the eternal aspects of the universe. All believed that the non-rational soul comes into being and unavoidably perishes. Some, like al-Farabi, believed that the rational soul may or may not survive eternally; others, like Ibn Sina, believed that it has no beginning and no end; still others, such as Ibn Rushd, believed that the soul with all its individual parts comes into existence and is eventually destroyed.
The existence of the soul
All Muslim philosophers concerned themselves with the subject of the soul. The most detailed and most important works on this subject are those of al-Kindi, al-Farabi, Ibn Sina and Ibn Rushd. Muslim philosophers recognized that the first issue, that confronts the human mind with regard to the soul is its existence. That is why, at the very beginning of his inquiry about the soul in al-Shifa’ (Healing), Ibn Sina asserts that we infer the existence of the soul from the fact that we observe bodies that perform certain acts with some degree of will. These acts are exemplified in taking nourishment, growing, reproducing, moving and perceiving. Since these acts do not belong to the nature of bodies, for this nature is devoid of will, they must belong to a principle they have other than bodies. This principle is what is called ‘soul’.This argument is intended to prove the existence of the animal soul, which includes the plant soul. The soul is the source of acts performed by the will, not inasmuch as it is ‘a substance’ (an independent entity), but inasmuch as it is ‘the principle of such acts’. The rational soul, on the other hand, need not look outside itself to infer its existence. It is aware of its existence with immediacy, that is, without any instruments. Ibn Sina’s example of the suspended man is intended to prove that the rational soul is aware of itself apart from any body. His argument boils down to the view that, even if the adult rational soul is not aware of anything material, not even its body, it remains aware of its own existence.