In order to understand whether the soul has an independent existence, we should examine the nature of human knowledge. It is the essential property of matter that it cannot assume two different forms simultaneously. To transform a silver spoon into a silver glass, it is necessary that the spoon-form as such, should cease to exist. This property is common to all bodies, and a body that lacks it cannot be regarded as a body. Now when we examine the nature of perception, we see that there is a principle in man which, in so far as it is able to know more than one thing at a time, can assume, so to say, many different forms simultaneously. This principle cannot be matter, since it lacks the fundamental property of matter. The essence of the soul consists in the power of perceiving a number of objects at one and the same moment of time. But it may be objected that the soul-principle may be either material in its essence, or a function of matter. There are, however, reasons to show that the soul cannot be a function of matter.
(a). A thing which assumes different forms and states, cannot itself be one of those forms and states. A body which receives different colors should be, in its own nature, colorless. The soul, in its perception of external objects, assumes, as it were, various forms and states; it, therefore, cannot be regarded as one of those forms. Ibn Maskawaih seems to give no countenance to the contemporary Faculty-Psychology; to him different mental states are various transformations of the soul itself.
(b). The attributes are constantly changing; there must be beyond the sphere of change, some permanent substratum which is the foundation of personal identity.
Having shown that the soul cannot be regarded as a function of matter, Ibn Maskawaih proceeds to prove that it is essentially immaterial. Some of his arguments may be noticed:—
1. The senses, after they have perceived a strong stimulus, cannot, for a certain amount of time, perceive a weaker stimulus. It is, however, quite different with the mental act of cognition.
2. When we reflect on an abstruse subject, we endeavour to completely shut our eyes to the objects around us, which we regard as so many hindrances in the way of spiritual activity. If the soul is material in its essence, it need not, in order to secure unimpeded activity, escape from the world of matter.
3. The perception of a strong stimulus weakens and sometimes injures the sense. The intellect, on the other hand, grows in strength with the knowledge of ideas and general notions.
4. Physical weakness due to old age, does not affect mental vigour.
5. The soul can conceive certain propositions which have no connection with the sense-data. The senses, for instance, cannot perceive that two contradictories cannot exist together.
6. There is a certain power in us which rules over physical organs, corrects sense-errors, and unifies all knowledge. This unifying principle which reflects over the material brought before it through the sense-channel, and, weighing the evidence of each sense, decides the character of rival statements, must itself stand above the sphere of matter.