As for the rational soul

The third of them is the view of those who hold spontaneous generation (al-tawallud al-dhātī), and of this view are Dr. Bastian of England and Professor Haeckel (1) of Germany. Life, on their doctrine, is from the non-living, and accordingly it is not eternal.

(As for the Rational Soul)

It is, among the spiritualists (al-rūḥīyīn), the source of the intellectual faculties; but among the materialists it arises from cerebral operations, the brain being the source of the intellectual faculties; and they deny its immateriality (tajarrud) in the sense that the spiritualists establish. By way of exposition and clarification of their doctrine we adduce an example of the manner of the brain's action. When the effect of the etheric vibration reaches the retina of the eye from the object seen, it produces a vibration in the optic nerve; and this etheric vibration extends to the layer settled in a particular place in the brain and reaches a sensitive cell among the cerebral cells; and here this cell sets about converting this effect into a visual sensation. So likewise with the remaining cerebral operations of the external and internal senses. The sum, then, of the cerebral acts arising from the vibrations in the ether and the nerves and from the arousal of the cells by their motion, which is present in the cerebral substance, is the soul. Accordingly these acts arise from the force latent in matter and from matter's discharging of that force latent in its own essence. And the upshot of the view is that the intellect is a motion in the fine fibres of the brain.

That the philosopher Naṣīr al-Dīn al-Ṭūsī mentioned, in the course of his treatise, certain effects of the external and internal senses, and that he pointed to the imprinting of forms in the brain, has called upon us to mention something of the composition of the brain and the operations of its vesicles (ḥuwayṣilāt) according to what the men of learning of this age have established, resting upon sound observation and express discovery, so that the introduction may thereby be adequate to the purpose and clarifying for the investigations of the treatise. And God Most High is the patron of success.

(The Brain)

It is among the most important of the nerve centres, and it is composed of a medullary substance which occupies the skull. They reckon its weight in man at approximately (1300) grammes. It is made up of several parts, the most important of them four:

(1) The cerebrum, which occupies the upper and forward part of the skull, and its weight amounts to about nine tenths of the weight of the whole brain.

(2) The cerebellum, whose seat is at the base of the cerebrum on the posterior side.

(3) The pons, which is a broad nerve girdle that winds about the medulla oblongata and joins the right part of the cerebellum to its left part.

(4) The medulla oblongata, which is the link between the brain and the spinal cord.

(The Cerebrum)

It is composed of two distinct substances. One of them is a dense white pith, and it is the fibrous nerve substance which is composed of the greater part of the brain and is composed of a vast portion of the nerve centres; it is the principal part of the nerves that connect the cerebral centres with the tissues. The second is superficial, thin, ash-grey in colour inclining to red, and it is the vesicular nerve substance; in it are the centres of sensation and the faculties of the intellect, such as thought and will, and in it are many corrugations known as the cerebral convolutions. The cerebral convolutions are of the greatest importance for the intellectual faculties and the nervous operations.

(The Cerebellum)

It is made up of three parts: two of them equal on the two sides, and one of them smaller than either. Like the cerebrum it is composed of a grey substance that is apparent and another, white, that is inward; and the first substance ramifies among the parts of the second. The function of the cerebellum is the regulating and ordering of the bodily movements; it has no concern with the movements themselves, for these are of the cerebrum's doing.

(The Medulla Oblongata)

It is a nervous mass binding the brain to the spinal cord, and it is made up of two substances, one of them white and apparent and the other grey and inward, the reverse of the cerebrum. In it many of the nerves of the spinal cord that come to the brain meet one another, and whenever these nerves reach it their direction is reversed: the nerves coming from the right side in the body go to the side of the left cerebral hemisphere, and the nerves coming from the left side go to the right cerebral hemisphere. It is one of the centres of the reflex operations, and the sole intermediary between the nerves of the spinal cord and the cerebrum and the cerebellum. Impressions are conveyed to the brain and the spinal cord by means of the nerves, which are threads of the utmost fineness and cords of connection joining the nerve centres one to another and distributed through the various regions of the body; they carry force from the centre to the regions of the body, and likewise convey impressions from it to the centre. Their function is twofold, and in them are two kinds of fibre. The first: the afferent fibres, and these are the nerves of sensation which convey impressions from the periphery to the centre. The second: the efferent, and these are the nerves which convey impressions from the centre to the periphery. In this action of theirs they have no power to generate the nerve current, but must needs have a stimulus in order to perform their function. The nerves of sensation are ordinarily stimulated by means of the external bodies which act upon their extremities, while the nerves of the centre are stimulated by means of the will, or by another force generated in the nerve centres. Accordingly the cerebral vesicles do not enter into operation unless a force from without has stimulated them; and each of them has a field whose breadth is in proportion to the number of nerve fibres that join it to the organs of the body and join them one to another.

(On the External and Internal Senses)

The sages of Islam and the philosophers of the Greeks held that the faculties of the external senses are lodged in the very instruments proper to them: the seeing faculty is lodged in the eye, the hearing faculty in the ear, and so too the other three faculties, without the cerebrum, its vesicles, and the nerve fibres that bind the centres one to another having any part in their action. Ibn Sīnā and Aristotle and others mentioned that in their compositions. But when the dawn of science broke in the latter centuries, the science of anatomy appeared in a clear new guise, and to it were joined the medical experiments, so that the men of learning obtained well-founded knowledge; and the conjoining of the two sciences came to be the issue of a science known as the science of (psychology) (Byscologie), (physiology) (Physiologie), or the psychical philosophy built upon the knowledge of the organic functions. This new guise began from the time of (Broca) (1), who discovered the centre of speech in the brain; and after him the discoveries followed one upon another and the knots were untied one after another, so that the boundaries within the brain and the functions of every point became known. With the appearance of this new foundation the men of learning of this age came to know that the action of the senses, whether external or internal, arises only from the action of the cerebral vesicles and the convolutions present in them and the nerve fibres that bind the centres one to another.

(On the Manner in which Percepts Arise)

The manner in which percepts arise and their tracings are impressed in the brain is this: that the brain, in the first of its phases, is without any concern for external stimuli, on account of the weakness in it of the grey substance. Then, as a man advances in the traversing of the stages of life and the circle of what he witnesses widens, the vesicles of sensation and motion take to growing, and it begins to train itself in the work by self-cultivation; so it takes to perceiving little by little, that is, its structure becomes prepared to receive the forms of the percepts that come to it from without, until, when its structure is completed and it has reached the degree of material formation, it becomes like the face of a mirror in which forms are reflected: so that when a stimulus acts upon it, it takes up the tracing of it and its tissue preserves it. The explanation of this is that the undulation of the external stimuli falls upon the five senses, and is transmitted by their nerve cords to the brain, where it ends at a sensitive cerebral cell; and here this cell sets about converting the arriving effect into a particular sensation, in a manner before whose veil science has halted without uncovering it. So what is conveyed from the optic nerve turns into a visual sensation, and what is conveyed from the auditory nerve turns into an auditory sensation; and on this pattern is the action of the other senses. And this action — that is, the action of the senses — occurs from the undulation, and is itself a kind of undulation: so what is conveyed from the eye turns into a visual undulation, and what is conveyed from the ear turns into an auditory undulation (etc.). At that point the work of the undulation conveyed by the nerve cords halts, and its force lies latent in the tracing conveyed from it and remains in the tracing in a state of slowness and latency, being in perpetual readiness to recover its force through the effect of new external sensibles, whether direct or indirect. On that account there are in the grey layer as many tracings as there are effects coming to it, and these are tracings of touch, hearing, sight, smell and taste.

Nor are the vesicles confined to the preserving of the tracings of sensation; they also store up the tracing of motion, which occurs from the undulations issuing from the movement of the muscles, the joints and the limbs, from walking, writing and their like — so that all the tracings are the result of an external sensible undulation which turns into two kinds, sensory and motor. The store in which the tracing is stored up is called the thinking faculty (al-mufakkirah) or the memory (al-dhākirah), these being two synonymous names used for one meaning, namely the arousal of forms preserved in the cerebral vesicle: when it is aroused by the action of a new external stimulus it turns into an acting force, and there arises what is called conception (taṣawwur) — that is, conception is the arousal of a form among the forms of the thinking faculty, as is made clear in the following example:

If lightning flashes and thunder follows upon it, the lightning acts upon the eye and the thunder upon the ear; that is, the apparatus of visual reception receives the external undulations by means of the optic nerve and registers them in its thinking faculty, that is, in a vesicle or several cerebral vesicles appointed and set apart for the reception of this effect; and the apparatus of auditory reception receives the undulations by means of the auditory nerve and preserves them in the vesicles set apart for its reception. So when the rumble of a new thunder suddenly strikes the ear, its vibration awakens directly the auditory forms — and this is the conception of the thunder — and indirectly, and that is the conception of the lightning. Thus there occur at one and the same moment two separate arousals of two separate forms, and two distinct conceptions, which are the thunder and the lightning. Conception here, then, attaches to a single trace of recollection, that is, to a single form, and not to many forms; and the thunder and the lightning are disclosed only by a single external sensible vibration, and they have but one form and one trace of recollection: for the thunder, sound, and for the lightning, light.

As for the linkage of the auditory conception with the visual conception, it is the issue of the linkage of the two centres by the nerve fibres, which make them share in a set of operations.

Memory

After the external effects reach the brain by means of the nervous vibrations, their forms are taken up in it and are traced in its vesicles; and this comes about with speed and ease or with slowness and difficulty, and the tracing is distinct and clear or weak and confused, and it holds firm for a long period or is swift to vanish — all of this in proportion to the particular structure of the vesicles and their natural preparedness; for if they are weak in structure and of little growth, they take up the tracings slowly, and these are of little clarity and permanence. What results from the taking up of the cerebral tracings, from their being stimulated and their return to appearance, is what is called memory. Accordingly memory is the totality of what results from multiple tracings and from their preparedness; it is not a single independent cerebral function having a particular delimited place, as the ancients and the philosophers of Islam, such as Ibn Sīnā and Ibn Rushd al-Andalusī (1) and others, held. Rather the singular name was applied to it only to delimit an operation shared among a set of memories; and there cannot be a single memory, because there is no single result for the conception of the multiple recollections and their preparedness, but rather many memories whose boundaries are dispersed among the vesicles of hearing, sight, smell, taste and touch; and it is proper that a name be assigned to each of them, such as the auditory, the visual, etc.

— Notes —

[23] (Jean Paul) is a German writer. He was born and died in the years — 1763 — 1825

[23] (1) (Hegel), a celebrated German philosopher and researcher, born in the year 1770 — 1831

[27] (1) Paul Broca (Paul Broca), a skilled French surgeon, born in Sainte-Foy-la-Grande, (Sainte-Fay-la-grande). He is one of the members of the medical academy, and he founded a great medical school. He was born and died in the years 1824 — 1880

[30] (1) Averroes 1126 — 1198 He is Abū al-Walīd Muḥammad ibn Aḥmad ibn Muḥammad ibn Rushd al-Qurṭubī, one of the greatest of the philosophers of Islam, the commentator on the philosophy of Aristotle. He attained the summit of his glory in the time of Yaʻqūb al-Manṣūr bi-Allāh; then his enviers slandered him, saying that he professed a philosophy at variance with the religion of Islam and that he deified Venus. So al-Manṣūr convened an assembly of the notables of Qurṭubah, and he was sentenced to banishment in the city of (Lūsāniyā); yet he returned by al-Manṣūr's decree, and he honoured him. He has many compositions which the Europeans translated from Arabic into the Western languages; and the philosopher (Ernest Renan) investigated his philosophy and the history of his life in his book which has been translated into Arabic