The Five Senses and How They Perceiveby Avicenna
As to the seeing power, philosophers have differed on the question of How they perceive.
In the Timaeus, Plato asserts that they perceive wholly and solely through a ray that shoots out beyond the eye. This makes them sense the sensible objects.
Others assert that the perceiving power itself encounters the sensible objects that are seen, and so perceives them.
Aristotle says that visual perception is caused by the intervening transparent body becoming effectively transparent by the light shining on it. This leads to an impression of the outspread (flattened) individual of such seen objects into the crystalline lens of the eye. This is similar to a pictorial impression as is effected in mirrors.
Aristotle is correct and Plato’s view is false*. If a ray goes out from the seat of sight and encounters sensible objects, then sight would not need light. It would:
- be able to perceive in the dark
- light up the air on its exit into the dark
*Superphysics note: Plato’s ‘ray’ is merely the aethereal willpower of the soul to want to perceive such objects. An example is curiosity.
Moreover such a ray will not fail of 1 of 2 modes. Either it will:
- subsist throughout the eye only, in which case Plato’s opinion that it goes forth from the eye is wrong; or
- subsist throughout a body other than the material of which the eye is composed; for it must inevitably have a vehicle to carry it, seeing that a ray is an accidental quality or mode, and furthermore seeing that that body which is other than the eye will not fail, in its turn, of being, either, firstly, sent out from the eye, in which case it will follow as a matter of course that the eye will not see all that is beneath the clear blue of the sky, since one body will not penetrate throughout the whole of another body, unless forsooth it moves the latter away and occupies its place;
Even should the disputer plead a vacuum, not only does Plato deny the existence of a vacuum utterly, but also if we accomodatingly yield this point and admit the existence of a vacuum, yet for all this the body that goes forth from the eye will penetrate throughout the body of water, for example, into such of its pores as are empty only, and not into the whole of the water’s bulk; so that even according to this opinion it will necessarily so be that the eye will see only some places of all that is under water;—or else, secondly, that body which is other than the eye will not fail of being an intervening body intermediate between the seer and the seen, in which case the light[I] which comes forth from the eye will subsist through it; nevertheless this opinion too is unsound, for the reason that every thing whatsoever is, in proximity to its source, so much the stronger, and in this respect light has not its equal; whence it follows as regards the object seen that, however closely and nearly it approaches to the eye, our perception will then be stronger;
Thus if we do away with the intermediary body, the eye will still perceive the object felt by its sense of sight, and thus the intermediary which is the vehicle and carrier of light is no longer needed, save accidentally (by chance); and then too there is no need, in order to see, for an exit of light: this too is a falsehood. Wherefore Plato’s opinion is worthless.
As for such as hold that the perceiver of the thing seen is the imaginative power itself through the imprinting of the form (image) of the sensible object upon it, these render the absent on the same footing as the present, since in the imaginative power there may exist the image of a[Pg 54] sensible object, notwithstanding the absence afterwards of the object that had been so felt: at which time however the living being so preserving that image will not be qualified with sight but with imagination and memory.
These theorists make a greater blunder. They see that they render a thing of Nature’s make and composition wholly idle, useless, and unneeded in the operation of visual perception; inasmuch as in their opinion the imaginative power itself meets immediately sensible objects, and thus spares Nature the task of adapting an instrument (organ), to wit the complex eye.
Wherefore the sound theory is that the configurations of things stand out in the transparent ambient—if it be effectively transparent on the shining of a luminant upon it—and hence they do not appear but in a polished body capable of receiving them, such as mirrors and the like; and so too there is in the eye a crystalline lens (or humor) into which the forms (pictures) of things are imprinted, just as their impression into mirrors; and in it, i.e., the lens or the eye, has been fitted up the seeing power; so that, if such forms are imprinted in it, it perceives them. Moreover, the objects of perception belonging in truth and deed to sight are the Colors.
The hearing power hears only sound.
Sound is a motion of air that the ear feels on 2 hard smooth bodies coming quickly close up one to the other, the escaping of the air from between them, its striking the ear, and its moving the air that is kept ready within the instrument (organ) of hearing.
Thus, if this inside air move the instrument, and if this instrument’s motion act upon the nerve of hearing, the hearing power (sense, faculty) perceives it in the measure of the strength or weakness of that motion. Indeed hardness is a conditio sine qua non; for, in the case of two soft bodies, the air will not escape from them, but will dissipate itself throughout their pores.
Smoothness too is just such a condition; because, in the case of rough (unsmooth) bodies, not the whole of the air will escape from between them suddenly and violently, but will be witheld (shut up) in the passages. And rapidity of contact also is a like condition; for if it come about gently and slowly, the air would not escape violently.
The echo too will arise from the rebound of the air escaping from between the two encountering bodies by reason of its hitting (slapping) against another hard, flat or hollow body filled with air, because of the air that is within it hindering the penetration of the escaped air, and the latter’s striking the ear [again] after the first stroke; on the same wise as in the first instance.
As to the Smelling Power; it smells odors on the sniffing in of air that has received its odor from an odoriferous body, as one body re[Pg 56]ceives its warmth from another warm body. Thus, if an animal snuffs up air like this into its nose until such air touches the front of the brain, and alters it to its own odor, the smelling power feels it.
This arises only on the coming to pass of the following change: When the moisture of the tasting instrument (organ)—to wit the tongue—becomes transformed into the juice of the newly-come food; and when the mass of this instrument (organ) has received that juice, the tasting power will perceive what has happened within the instrument.
This will only arise upon the organ’s (instrument’s) receiving the quality of that which is touched, and upon the touching power’s perceiving what has been thus presented (offered) within the organ.
Furthermore, simple sensibles, that are at once primary and as such the bases of all others, are in pairs, of which there are eight; and if we make each into singles, they become sixteen, to wit:—
Touch, four pairs:
- heat and cold
- moisture and dryness
- roughness and smoothness
- hardness and softness.
The four remaining senses, each have a pair:
- Smelling has fragrant and stinking
- Tasting has sweet and bitter
- Hearing has heavy sound and sharp sound (or dull and shrill)
- Sight has white and black
All other sensibles are made up from these simples, and are intermediates between some two of them, as for example grey (dusty color) from white and black, lukewarm from hot and cold. Moreover all sensibles are felt wholly and solely through a sort of gathering and sundering, shrinking and spreading; except sounds, which are felt only through sundering. Thus:—
[Warmth is felt through sundering] Cold is felt through gathering Moisture, through spreading Dryness, through shrinking Roughness, through sundering Smoothness, through spreading Hardness, through repelling, which is a sort of gathering and shrinking Softness, through being repelled, which is not devoid of spreading and sundering Sweetness, through spreading, devoid of sundering Bitterness, through sundering and shrinking Fragrant Odor, through spreading, devoid of sundering Stinking Odor, through sundering and shrinking Whiteness, through sundering Blackness, through gathering [15. and 16. Sounds: one pair, as above under “d.”]
As to the media (intermediaries) between the feeling powers and the felt forms, they are themselves devoid of the forms of sensibles; otherwise it would not be possible for them to be media, since their own forms—if they had any—would then so engage the apposite power as to divert it from perceiving any other forms. Such voidness or freedom from forms is either voidness wholly and altogether, or else relative voidness through equableness of the forms in the media, such as the equable proportion of the qualities touched in meat, which is a medium between the touching power and the quality touched, although meat is incontestably made up of qualities that are touched, yet notwithstanding this the equableness of the qualities has annihilated the forms in it.
Examples of the first division—absolute voidness and freedom from form—are the freedom of air, of water, and of what resembles them among the various media of sight, from color; the freedom of air and of water, both which are the two mediums of smelling, from odor; the freedom of water, which is the medium of tasting, from flavor; and the steadiness of the air, which is the medium of hearing, and its freedom from motion.
each of these powers, to wit the[Pg 59] five senses, if actually functionating, perceives only through coming into relation with the object felt, nay rather it only perceives at first so much as has been traced in it of the form of the object felt.
Thus, the eye only perceives that form which has imprinted itself in it of the object felt; so also the remainder of the powers (or senses).
Again, in the case of strong wearying sensibles, such as a loud noise, a strong smell, a shining and a flashing light, if they are repeated upon the organ (instrument), spoil and dullen it through their overworking it.
Each one of the 5 senses perceives, through the means of its own rightful perception and besides the same, five other things:
- rest (quiet)
That sight, touch, and taste perceive them, is evident. As to hearing, it perceives, in accordance (pursuance) with the variety of the number of sounds, the number of the sound-emitting objects; and, through the strength of the sounds, it perceives the size of the two objects that are hitting against each other; and, in accordance with a kind of change and fixedness of the sounds, it perceives motion and rest; and, in accordance with their volume around the sound-emitter, be the latter solid or hollow, it perceives some sorts of shapes.
As to smelling, it knows, in accordance with the change of directions whence the odors are emitted and reach it, and through the variety of these odors[Pg 60] in their qualities, it knows I say the number of the things smelt; through the measure of abundance of the smells, the size of such things; through the measure of proximity and distance, changeableness and fixedness, it recognizes their motion and their rest; and, in accordance with the sides on which their odor reaches it from one and the same body, it knows their shape. Still, these discriminations are very weak in this power among mankind, owing to the weakness of the power itself in the human race. [For all this, men have not the keen scent that many other animals have, and therefore such discriminations are in men very weak.]
 Plato’s Dialogue called Timaeus 45.
The names of the different parts of the eye are:
al-tabaqah al çalbah = sclerotica, hard-coat »»al-mashîmiyyah = choroid, vascular skin al-ghashâ-al-shabaky = retina, net skin al-ratûbah al-zajâjiyyah = glassy moisture al-ratûbah al-jalîdiyyah = crystalline lens »»»´ankabûtiyyah = ciliary, fibrous, hairy web al-hadaqah = pupilla al-tabaqah al-´inabiyyah = berry, grape coat qarniyyah = cornea al-multahimah = conjunctiva. [I]perhaps we ought to read «the ray».