A Compendium on the Soul · Avicenna
SECTION SIXTH
Moreover such a ray will not fail of one of two 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 else it will 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; and 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; and 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 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. Furthermore these theorists (opiners) make a greater blunder still, seeing 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.
As for the Hearing Power: it hears only sound. And sound is a motion of air that the ear feels on two 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 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.
Further, each of these powers, to wit the 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. Again, each one of the five senses perceives, through the means of its own rightful perception and besides the same, five other things, to wit: 1. shape; 2. number; 3. size; 4. motion; 5. 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 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.]
Ideas discussed in this chapter
Text from Project Gutenberg, public domain in the United States.