The nature of the soul
While Islam made it incumbent on Muslim philosophers to occupy themselves extensively with the study of the soul and to make certain statements that in some cases appear consistent with Islamic beliefs, Greek philosophy had the upper hand in forming the real convictions of Muslim philosophers with regard to the nature of the soul. Unless otherwise specified, reference to :he soul here is limited to the terrestrial soul to the exclusion of the celestial one, since Muslim philosophers concerned themselves primarily with the former. It must be pointed out at the outset that ‘soul’. (nafs) was used in more than one sense in Islamic philosophy; the term was used to refer to the plant or vegetative part of a living being, the animal or sensitive part, the rational part and finally the totality of all three parts. The first two are the non-rational soul and the totality is the human soul. To add to the confusion, ‘human soul’ is used only in the sense of this fourth type of soul. The plant, animal and; rational souls are also called powers or parts of the; soul. Only from the context can one understand, whether a Muslim philosopher was using ‘soul’ in the broad sense to mean the human soul (the totality of the parts of the soul), or in the narrow sense to mean a specific part of the human soul.
Inasmuch as it has a certain relation to a body, the soul is a form for that body, that is, the perfection of that body. It is a form because a natural body is composed of matter and form, which in the case of animals are body and soul. Since it has been shown that the soul is the source of will and therefore is not matter, it remains a form. Perfection is of two types, primary and secondary. A primary perfection is what makes a thing actually a species, as shape does for the sword, or a genus as sensation and-movement do for animals. A secondary perfection is an act necessitated by the nature of the species or genus, such as cutting for the sword and touching for animal. The soul is a primary perfection of a natural body capable of performing the secondary perfections necessitated by this primary perfection. Together with its body, the soul constitutes a material substance. This substance can be the subject of plant, animal or human life.
The soul is a perfection inasmuch as it makes a natural body into a plant, an animal or a rational being. However, to define the soul as a perfection does not give us a clue as to what the soul is in itself, but only inasmuch as it has a relation to the body. The body is, therefore, an essential element in the definition of the soul. Without relating to a body, the thing we call ‘soul’ is not a soul and does not require the body as an essential part of its definition. Note, however, that in spite of this assertion, perhaps for the lack of any better term, Muslim philosophers use ‘soul’ also to refer to the rational soul after it separates from the body and reaches a complete state of purity from matter.
In its first or lowest stages of relating to the body, the soul is the plant soul, which is a primary perfection for an organic natural body inasmuch as this body can take nourishment, grow and reproduce. The plant soul is the power human beings and other animals share with plants. If the body with a soul is an animal, the soul develops into the animal soul, which is a primary perfection for an organic natural body inasmuch as this body has sensation and movement through will. While this soul includes the plant soul, it has also a sensitive power and a locomotive one. The sensitive power has both external and internal senses. The external senses are, in priority of existence, touch, taste, smell, hearing and sight. The first three are said to be necessary for survival and the last two for well being. In Talkhis kitab an-nafs (Middle Commentary on Aristotle’s On the Soul), IBN RUSHD asserts that the five external senses may be in potentiality, as in infancy and sleep, or in actuality, as in daily seeing or hearing. He also argues that there cannot be any external sense other than these five because there would be no function for it, since there is no external sensation other than the objects of the five senses mentioned above. Most Muslim philosophers mention three types of internal senses: common sense, imagination and memory. IBN SINA enumerates five internal senses: common sense, representational power, imagination, estimative power and memory. On the whole, the philosophers agree on the function of the common sense, imagination and memory; the function that Ibn Sina limits to the representational and estimative powers, other Muslim philosophers allocate to the imagination.
The common sense is an internal power in which all the objects of the external senses are collected. Contrary to the external senses, which can grasp only one type of sensation, as sight grasps light and hearing grasps sound, the common sense can grasp all external sensations, such as that honey is of such and such a colour, texture and smell. The representational power preserves the sensations of the common sense even after sensible things disappear. The imagination selects at will to combine some of the objects of the representational power with each other and to separate the rest. It makes its judgment about external things, but in the absence of these things. That is why it functions best when the external senses, which represent external things, are not at work, as in sleep. Ibn Rushd points out that animals such as worms and flies that do not act except in the presence of sensible things are devoid of imagination. The imagination is called such inasmuch as it is an animal instrument; it is called cognitive inasmuch as it is a rational instrument. The estimative power grasps non-sensible notions of sensible things, such as the sheep’s notion that the wolf is to be avoided. This notion is about a sensible thing but is not grasped through the external senses, as is the colour or shape of a wolf. Memory preserves the notions of the estimative power. The imagination acts on the objects of memory in the same way it acts on those of the representational powers. Like the objects of the external senses, those of the internal senses are particular and material. The difference is that they can be experienced in the absence of external things and are to some degree abstracted from matter. The locomotive power branches into that which causes movement and that which actually moves. The former, the desiderative power, subdivides into the appetitive and the irascible. The appetitive causes movement toward what is imagined to be necessary or beneficial in the pursuit of pleasure. The irascible causes avoidance of what is imagined to be harmful or an impediment in the pursuit of dominance. The power that actually moves uses the nerves to relax the muscles at the demands of the appetitive power or tighten them at the demands of the irascible one.