University of California, Irvine
Instructor: Dr. Barbara J. Becker
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Now there are four chief obstacles in grasping truth,
which hinder every man, however learned, and scarcely allow any one to
win a clear title to learning, namely, submission to faulty and unworthy
authority, influence of custom, popular prejudice, and concealment of our
own ignorance accompanied by an ostentatious display of our knowledge....
--Roger Bacon (c.1210 - c.1292) |
By the end of the thirteenth century, many ancient texts had made their way into Latin translation. European scholars, like Roger Bacon, found it hard to ignore--let alone explain and reconcile--the differences and contradictions these works contained. Were some--or perhaps all--in error? If so, who was to blame: the original author or the translator? How could the diligent scholar identify and promote only "worthy" authorities while making sure "unworthy" sources were corrected or expunged?
Take a look at the following examples of what some early "authorities" had to say about the sense of sight.
__________
Empedocles (c.492 - 432 BCE)
And even as when a man, thinking to sally forth through a stormy night, gets him ready a lantern, a flame of flashing fire, fastening to it horn plates to keep out all manner of winds; and they scatter the blast of the winds that blow, but the light leaping out through them shines across the threshold with its unyielding rays inasmuch as it is finer; even so did love surround the elemental fire in the round pupil and confine it with membranes and fine tissues, which are pierced through and through with innumerable passages. They keep out the deep water that surround the pupil, but they let through the fire, inasmuch as it is finer.
Leucippus of Miletus (fl.430 BCE)
Every change produced by or impressed on things takes place by virtue of a contact; all our perceptions are tactile, and all our senses are varieties of touch. Consequently, since our mind does not proceed from within us to sally forth and touch external objects, it is necessary for these objects to come and touch our mind by passing through our senses. Now we do not see objects approaching us when we perceive them; therefore, they must be sending to our mind something which represents them, some shadow-like images or material likenesses which cover these bodies; these images move about on their surfaces, and can detach themselves in order to bring to our mind the shapes, colors, and all the other qualities of the bodies from which they emanate.
Plato (427 - 347 BCE)
Timaeus
And the first organs [the gods] fashioned were those that give us light, which they fastened there in the following way. They arranged that all fire which had not the property of burning, but gave out a gentle light, should form the body of each day's light. The pure fire within us that is akin to this they caused to flow through the eyes, making the whole eyeball, and particularly its central part, smooth and close-textured so that it would keep in anything of coarser nature, and filter through only this pure fire. So when there is daylight around the visual stream, it falls on its like and coalesces with it, forming a single uniform body in the line of sight, along which the stream from within strikes the external object. Because the stream and daylight are similar, the whole so formed is homogeneous, and the motions caused by the stream coming into contact with an object or an object coming into contact with the stream penetrate right through the body and produce in the soul the sensation which we call sight.
But when the kindred fire disappears at nightfall, the visual stream is cut off; for what it encounters is unlike itself and so it is changed and quenched, finding nothing with which it can coalesce in the surrounding air which contains no fire. It ceases therefore to see and induces sleep. For when the eyelids, designed by the gods to protect the sight, are shut, they confine the activity of the fire within, and this smoothes and diffuses the internal motions, and produces a calm; when this calm is profound the resultant sleep has few dreams, but when rather more motion remains images, corresponding in quality and number to the type and location of the residual motions, are formed internally and remembered as external events when we wake.
Aristotle (384 - 322 BCE)
On the Soul, Book II, Chapter VII
[L]ight is neither fire nor any kind whatsoever of body nor an efflux from any kind of body (if it were, it would again itself be a kind of body)--it is the presence of fire or something resembling fire in what is transparent. It is certainly not a body, for two bodies cannot be present in the same place. The opposite of light is darkness; darkness is the absence from what is transparent of the corresponding positive state above characterized; clearly therefore, light is just the presence of that....
Not everything that is visible depends upon light for its visibility. This is only true of the 'proper' color of things. Some objects of sight which in light are invisible, in darkness stimulate the sense; that is, things that appear fiery or shining. This class of objects has no simple common name, but instances of it are fungi, flesh, heads, scales, and eyes of fish. In none of these is what is seen their own 'proper' color. Why we see these at all is another question. At present what is obvious is that what is seen in light is always color. That is why without the help of light color remains invisible. Its being color at all means precisely its having in it the power to set in movement what is already actually transparent, and, as we have seen, the actuality of what is transparent is just light.
The following experiment makes the necessity of a medium clear.
If what has color is placed in immediate contact with the eye, it cannot
be seen. Color sets in movement not the sense organ but what is transparent,
e.g. the air, and that, extending continuously from the object to the organ,
sets the latter in movement. Democritus misrepresents the facts when
he expresses the opinion that if the interspace were empty one could distinctly
see an ant on the vault of the sky; that is an impossibility. Seeing
is due to an affection or change of what has the perceptive faculty, and
it cannot be affected by the seen color itself; it remains that it must
be affected by what comes between. Hence it is indispensable that
there be something in between--if there were nothing, so far from seeing
with greater distinctness, we should see nothing at all.
On Sense and the Sensible
Chapter I.
The senses which operate through external media, viz. smelling, hearing, seeing, are found in all animals which possess the faculty of locomotion. To all that possess them they are a means of preservation; their final cause being that such creatures may, guided by antecedent perception, both pursue their food, and shun things that are bad or destructive. But in animals which have also intelligence they serve for the attainment of a higher perfection. They bring in tidings of many distinctive qualities of things, from which the knowledge of truth, speculative and practical, is generated in the soul.
Of the two last mentioned, seeing, regarded as a supply for the primary
wants of life, and in its direct effects, is the superior sense; but for
developing intelligence, and in its indirect consequences, hearing takes
the precedence. The faculty of seeing, thanks to the fact that all
bodies are colored, brings tidings of multitudes of distinctive qualities
of all sorts; whence it is through this sense especially that we perceive
the common sensibles, viz. figure, magnitude, motion, number....
If the visual organ proper really were fire, which is the doctrine of Empedocles, a doctrine taught also in the Timaeus, and if vision were the result of light issuing from the eye as from a lantern, why should the eye not have had the power of seeing even in the dark? It is totally idle to say, as the Timaeus does, that the visual ray coming forth in the darkness is quenched. What is the meaning of this 'quenching' of light? That which, like a fire of coals or an ordinary flame, is hot and dry is, indeed, quenched by the moist or cold; but heat and dryness are evidently not attributes of light. Or if they are attributes of it, but belong to it in a degree so slight as to be imperceptible to us, we should have expected that in the daytime the light of the sun should be quenched when rain falls, and that darkness should prevail in frosty weather. Flame, for example, and ignited bodies are subject to such extinction, but experience shows that nothing of this sort happens to the sunlight.
Empedocles at times seems to hold that vision is to be explained as above stated by light issuing forth from the eye, e.g. in the following passage:--
As when one who purposes going abroad prepares a lantern,Sometimes he accounts for vision thus, but at other times he explains it by emanations from the visible objects.
A gleam of fire blazing through the stormy night,
Adjusting thereto, to screen it from all sorts of winds, transparent sides,
Which scatter the breath of the winds as they blow,
While, out through them leaping, the fire, i.e. all the more subtile part of this,
Shines along his threshold old incessant beams:
So [Divine love] embedded the round "lens", [viz.]
the primaeval fire fenced within the membranes,
In [its own] delicate tissues;
And these fended off the deep surrounding flood,
While leaping forth the fire, i.e. all its more subtile part--.
Democritus, on the other hand, is right in his opinion that the eye is of water; not, however, when he goes on to explain seeing as mere mirroring. The mirroring that takes place in an eye is due to the fact that the eye is smooth, and it really has its seat not in the eye which is seen, but in that which sees. For the case is merely one of reflexion. But it would seem that even in his time there was no scientific knowledge of the general subject of the formation of images and the phenomena of reflexion. It is strange too, that it never occurred to him to ask why, if his theory be true, the eye alone sees, while none of the other things in which images are reflected do so.
True, then, the visual organ proper is composed of water, yet vision appertains to it not because it is so composed, but because it is translucent--a property common alike to water and to air. But water is more easily confined and more easily condensed than air; wherefore it is that the pupil, i.e. the eye proper, consists of water. That it does so is proved by facts of actual experience. The substance which flows from eyes when decomposing is seen to be water, and this in undeveloped embryos is remarkably cold and glistening. In sanguineous animals the white of the eye is fat and oily, in order that the moisture of the eye may be proof against freezing. Wherefore the eye is of all parts of the body the least sensitive to cold: no one ever feels cold in the part sheltered by the eyelids. The eyes of bloodless animals are covered with a hard scale which gives them similar protection.
It is, to state the matter generally, an irrational notion that the eye should see in virtue of something issuing from it; that the visual ray should extend itself all the way to the stars, or else go out merely to a certain point, and there coalesce, as some say, with rays which proceed from the object. It would be better to suppose this coalescence to take place in the fundament of the eye itself. But even this would be mere trifling. For what is meant by the 'coalescence' of light with light? Or how is it possible? Coalescence does not occur between any two things taken at random. And how could the light within the eye coalesce with that outside it? For the environing membrane comes between them.
That without light vision is impossible has been stated elsewhere; but, whether the medium between the eye and its objects is air or light, vision is caused by a process through this medium.
Accordingly, that the inner part of the eye consists of water is easily intelligible, water being translucent.
Now, as vision outwardly is impossible without [extra-organic] light, so also it is impossible inwardly [without light within the organ]. There must, therefore, be some translucent medium within the eye, and, as this is not air, it must be water. The soul or its perceptive part is not situated at the external surface of the eye, but obviously somewhere within: whence the necessity of the interior of the eye being translucent, i.e. capable of admitting light. And that it is so is plain from actual occurrences. It is matter of experience that soldiers wounded in battle by a sword slash on the temple, so inflicted as to sever the passages of [i.e. inward from] the eye, feel a sudden onset of darkness, as if a lamp had gone out; because what is called the pupil, i.e. the translucent, which is a sort of inner lamp, is then cut off [from its connexion with the soul].
Hence, if the facts be at all as here stated, it is clear that--if one should explain the nature of the sensory organs in this way, i.e. by correlating each of them with one of the four elements,--we must conceive that the part of the eye immediately concerned in vision consists of water, that the part immediately concerned in the perception of sound consists of air, and that the sense of smell consists of fire....
Titus Lucretius Carus (c.96 - c.55 BCE)
On the Nature of Things (c.50 BCE), Book IV
lines 42 - 97
I say that effigies of things,
And tenuous shapes from off the things are sent,
From off the utmost outside of the things,
Which are like films or may be named a rind,
Because the image bears like look and form
With whatso body has shed it fluttering forth--
A fact thou mayst, however dull thy wits,
Well learn from this: mainly, because we see
Even 'mongst visible objects many be
That send forth bodies, loosely some diffused--
Like smoke from oaken logs and heat from fires--
And some more interwoven and condensed--
As when the locusts in the summertime
Put off their glossy tunics, or when calves
At birth drop membranes from their body's surface,
Or when, again, the slippery serpent doffs
Its vestments 'mongst the thorns--for oft we see
The breres [briars] augmented with their flying spoils:
Since such takes place, 'tis likewise certain too
That tenuous images from things are sent,
From off the utmost outside of the things.
For why those kinds should drop and part from things,
Rather than others tenuous and thin,
No power has man to open mouth to tell;
Especially, since on outsides of things
Are bodies many and minute which could,
In the same order which they had before,
And with the figure of their form preserved,
Be thrown abroad, and much more swiftly too,
Being less subject to impediments,
As few in number and placed along the front.
For truly many things we see discharge
Their stuff at large, not only from their cores
Deep-set within, as we have said above,
But from their surfaces at times no less--
Their very colours too. And commonly
The awnings, saffron, red and dusky blue,
Stretched overhead in mighty theatres,
Upon their poles and cross-beams fluttering,
Have such an action quite; for there they dye
And make to undulate with their every hue
The circled throng below, and all the stage,
And rich attire in the patrician seats.
And ever the more the theatre's dark walls
Around them shut, the more all things within
Laugh in the bright suffusion of strange glints,
The daylight being withdrawn. And therefore, since
The canvas hangings thus discharge their dye
From off their surface, things in general must
Likewise their tenuous effigies discharge,
Because in either case they are off-thrown
From off the surface. So there are indeed
Such certain prints and vestiges of forms
Which flit around, of subtlest texture made,
Invisible, when separate, each and one.
Again, all odour, smoke, and heat, and such
Streams out of things diffusedly, because,
Whilst coming from the deeps of body forth
And rising out, along their bending path
They're torn asunder, nor have gateways straight
Wherethrough to mass themselves and struggle abroad.
But contrariwise, when such a tenuous film
Of outside colour is thrown off, there's naught
Can rend it, since 'tis placed along the front
Ready to hand.
...since shape examined by our hands
Within the dark is known to be the same
As that by eyes perceived within the light
And lustrous day, both touch and sight must be
By one like cause aroused. So, if we test
A square and get its stimulus on us
Within the dark, within the light what square
Can fall upon our sight, except a square
That images the things? Wherefore it seems
The source of seeing is in images,
Nor without these can anything be viewed.Now these same films I name are borne about
And tossed and scattered into regions all.
But since we do perceive alone through eyes,
It follows hence that whitherso we turn
Our sight, all things do strike against it there
With form and hue. And just how far from us
Each thing may be away, the image yields
To us the power to see and chance to tell:
For when 'tis sent, at once it shoves ahead
And drives along the air that's in the space
Betwixt it and our eyes. And thus this air
All glides athrough our eyeballs, and, as 'twere,
Brushes athrough our pupils and thuswise
Passes across. Therefore it comes we see
How far from us each thing may be away,
And the more air there be that's driven before,
And too the longer be the brushing breeze
Against our eyes, the farther off removed
Each thing is seen to be: forsooth, this work
With mightily swift order all goes on,
So that upon one instant we may see
What kind the object and how far away.Nor over-marvellous must this be deemed
In these affairs that, though the films which strike
Upon the eyes cannot be singly seen,
The things themselves may be perceived. For thus
When the wind beats upon us stroke by stroke
And when the sharp cold streams, 'tis not our wont
To feel each private particle of wind
Or of that cold, but rather all at once;
And so we see how blows affect our body,
As if one thing were beating on the same
And giving us the feel of its own body
Outside of us. Again, whene'er we thump
With finger-tip upon a stone, we touch
But the rock's surface and the outer hue,
Nor feel that hue by contact--rather feel
The very hardness deep within the rock.
...our eye-balls tend to flee the bright
And shun to gaze thereon; the sun even blinds,
If thou goest on to strain them unto him,
Because his strength is mighty, and the films
Heavily downward from on high are borne
Through the pure ether and the viewless winds,
And strike the eyes, disordering their joints.
So piercing lustre often burns the eyes,
Because it holdeth many seeds of fire
Which, working into eyes, engender pain.
Again, whatever jaundiced people view
Becomes wan-yellow, since from out their bodies
Flow many seeds wan-yellow forth to meet
The films of things, and many too are mixed
Within their eye, which by contagion paint
All things with sallowness. Again, we view
From dark recesses things that stand in light,
Because, when first has entered and possessed
The open eyes this nearer darkling air,
Swiftly the shining air and luminous
Followeth in, which purges then the eyes
And scatters asunder of that other air
The sable shadows, for in large degrees
This air is nimbler, nicer, and more strong.
And soon as ever 'thas filled and oped with light
The pathways of the eyeballs, which before
Black air had blocked, there follow straightaway
Those films of things out-standing in the light,
Provoking vision--what we cannot do
From out the light with objects in the dark,
Because that denser darkling air behind
Followeth in, and fills each aperture
And thus blockades the pathways of the eyes
That there no images of any things
Can be thrown in and agitate the eyes.
Alexander Aphrodisiensis (c.100)
On the Soul
Some people explain vision by the stress of air. The air adjoining the pupil is excited by vision and formed into a cone which is stamped on its base by an impression of the object of vision, and thus perception is created similar to the touch of a stick.... And with regard to the assumption that vision originates in the hegemonikon, [literally, "guiding thing"; an individual's decision-making center; the mind] if here, too, the theory of tensional motion holds, as they maintain, would there not occur certain interruptions of seeing, since tension would not be produced continuously on the boundaries and neither, consequently, impact? The same question could be asked about what happens when we touch other bodies. For here no interruptions of perception occur, as ought to happen. For such is their theory of tensional motion. If they would argue--as they actually do not--that only the pneuma which they call vision (and not the other senses) performs the said movement called tensional, it would be absurd. On the whole the doctrine of tensional motion is full of paradoxes. This motion represents a uniform primary self-mover which has been shown impossible by those examining the simple movements. Further, if there is one thing which holds together the whole cosmos with all its contents, and for every single and particular body there is something which holds it together, would that not necessarily imply that the same thing carries out simultaneously opposite motions?.... Moreover, granted that vision originates through a pressure in the air between (the eye and the object), that would obviously lead to the perception of hardness and softness, roughness and smoothness, moisture and dryness rather than to that of color. Those (qualities) we can perceive also by a stick, but not colors or shapes or quantity nor anything connected with magnitude, all of which are visible things. But it is colors and shapes and magnitude in particular which are perceived by vision. On the whole this would mean that vision is some kind of touch.
Further: why can objects in the dark not be seen from a place in the light, while from the dark one can see objects which are in the light? For can one believe the doctrine that illuminated air becomes more powerful because of the mixture and can propagate the sensation through pressure, whereas dark air is slack and cannot be stressed by vision, in spite of being denser than illuminated air?.... It is further a fact that two illuminated houses standing opposite each other can be seen by their inhabitants not less even though the air between them is dark and therefore not stressed.
Galen (131 - 201)
On the Doctrines of Hippocrates and Plato
Since of all the sensations only the sense of vision receives the perception of the object by transmission through the medium of air, not as from a stick but as some part kindred to and coalescing with it, and only by this and by the incidence of light does it get the specialty of seeing, it stands to reason that it needs the luminous pneuma which flows from above, strikes the surrounding air and assimilates it to itself....
That we see through the medium of air is obvious and agreed by all. However, the question is whether this happens as if something comes to us along a path from the object, or if the air serves us as an organ of cognition for an object seen just as the nerves do for an object touched. Most people believe that the modification produced by the things that reach us is transmitted by the nerve to the ruling part of the soul, and thus the object is perceived. But these people do not realize that the sensation of pain could not arise in a limb cut or broken or burned, if the limb itself did not hold the faculty of sensation. But actually the opposite of their opinion is true. The nerve itself namely is part of the brain, like a branch or offshoot of a tree, and this part into which the nerve is rooted fully receives the faculty of sensation and thus becomes able to distinguish the object touching it. Something similar happens to the air surrounding us. When illuminated by the sun it becomes an organ of vision precisely as the pneuma arriving (in the eye) from the brain, but before the illumination occurs which produces a modification through the incidence of the sun's rays, the air cannot become such an affected organ.
Hunain ibn Ishaq (810 - 877) [Johannitius]
Ten Treatises on the Eye
All people acknowledge and agree that we see only by the hole which is in the pupil. Now if this hole had to wait until something coming from the [visible] object reached it, or a power..., or a form, an outline or a quality, as some people maintain, we should not know, in looking at an object, either its extent or its volume.... Its entering into the eyes is something which reason does not comprehend and of which nobody has ever heard, for according to this hypothesis a complete form or outline of the viewed object would necessarily reach and enter into the eye of the beholder at the same moment. Supposing then that a great many people looked at it, say, for example, ten thousand persons, it would have to return to the eye of every one of them, and its form and outline would have to enter completely into them. But this is far from probable and must therefore be ranked among the untenable hypotheses.
Roger Bacon, (c.1210 - c.1292)
Opus Majus (c.1268), "Concerning Optics," First Part
Second Distinction, Chapter I.
Clearly, therefore, in order that there may be no scruple of doubt in what follows, the structure of the eye must be studied, because without this nothing can be known concerning the method of vision. But some writers say less, some more, and in certain things they are at variance.... Therefore their discussion is in itself obscure, nor can we understand it unless we have recourse to the fuller treatment of the subject by the writers on medicine and on natural philosophy.... I hope that these facts can be made clear.... But that I may not draw upon sources of individual opinions to too great an extent, I shall confine my description of the structure of the eye mainly to three authorities--Alhazen (965 - c.1040) in his first book on Perspective, Constantinus (fl.1065 - 1085) in his book On the Eye, and Avicenna ((980 - 1037) in his books; for these writers are sufficient and they treat more definitely the matters in which we are interested. I cannot, however, give the exact words of each, because they are sometimes at variance owing to faulty translation, but from them all I shall form a single statement of the truth....
As we now understand those things that need explanation on account of the mode of our vision, we must pass on to a consideration of the mode itself and how vision takes place. First we must determine what is required for rectilinear vision. The first requisite here considered is, that vision needs the impression of a visible object, for without this there can be no vision. Accordingly Aristotle says in the second book On the Soul that in every case the sense receives the impressions of sensible things, so that there may be an act of sensation.... [W]hen there is an obstacle between the impression of the object and vision, vision does not take place. But when every impediment is removed, so that the impression comes to the eye, the object is seen. Wherefore vision must happen by means of an impression; but especially by means of the impression of light and color.... Without contradiction we find by experiment that without light nothing is seen....
Fifth Distinction, Chapter II.
We must understand that vision is not completed in the eyes, according to the teaching of the authors on Perspective.... For in two eyes there are different judgments. Therefore one object as viewed will be considered as quite different. Therefore there must be something sentient besides the eyes, in which vision is completed and of which the eyes are the instruments that give it the visible species. This is the common nerve in the surface of the brain, where the two nerves coming from the two parts of the anterior brain meet, and after meeting are divided and extend to the eyes....
Fifth Distinction, Chapter III.
[T]he eyes not only judge concerning a visible object, but ... the judgment is begun in them and is completed by ... the visual faculty with its source in the common nerve.... [T]herefore the visual act is a single and undivided one, which is performed by the eyes and the common nerve.
John Pecham (c.1230 - 1292)
Perspectiva Communis (c.1278), Book I
Proposition 44.
By assuming that sight occurs through rays issuing from the eye, mathematicians exert themselves unnecessarily.
For the manner in which vision occurs is adequately described above, by which [description] all the phenomena of vision can be saved. Therefore, it is superfluous to posit such rays. I say this following in the footsteps of the author of the Perspectiva [Alhazen]. Nevertheless, Alkindi (c.801 - c.866) teaches differently [in his treatise] De aspectibus, the Platonists have judged otherwise, and philosophers are seen in many places to understand differently. Augustine (354 - 430), who declares that the power of the soul has an effect on the light of the eye in a manner different from any that has hitherto been investigated, also teaches otherwise.
Rays issuing from the eye and falling on a visible object cannot suffice for vision.
If it should be supposed that rays issue from the eye and fall on the visible object as if to seize it, either they return to the eye or they do not. If they do not return, vision is not achieved through them, since soul does not issue from body. If they do return, how do they do so? Are they animated? Are all visible objects mirrors by [virtue of the property of] reflecting rays? Furthermore, if the rays return to the eye with the form of the visible object, they go out in vain, since light itself (or the form of the visible object through the power of light) diffuses itself throughout the whole medium. Therefore, the visible object need not be sought out by rays as by messengers. Moreover, how would any power of the eye be extended all the way to the stars, even if the whole body were transformed into spirit?
The natural light of the eye contributes to vision by its radiance.
For, as Aristotle says, the eye is not merely the recipient of action but acts itself, just as shining bodies do. Therefore, the eye must have a natural light to alter visible species and make them commensurate with the visual power, for the species are emitted by the light of the sun but moderated with respect to the eye by mixing with the natural light of the eye. This is why Aristotle said that sight occurs when the outward action [of the eye] is strong, [for] when the action entering the eye is strong, as with a solar ray, it overpowers sight; nor, of itself, can this inward action be made commensurate with sight. It is evident, therefore, that there is some kind of emission of rays, but not of the Platonic type such that rays emitted by the eye are, as it were, immersed in the visible form and then returned to the eye as messengers. But rays [emitted by the eye] do have some effect on sight in the manner described above. This is evident as follows. Since vision is of the same kind in all animals, and certain animals are able to bestow a multiplicative power on colors by the light of their eyes so as to see them at night, it follows that the light of the eye has some effect on [external] light. Whether it goes beyond this, I do not determine, save only by following in the footsteps of the Author, [Alhazen] as I have said before.
Witelo of Silesia (c.1230 - c.1275)
Perspectiva (1269), Book III, Theorem 5
It is impossible for sight to be applied to the visible object by rays issuing from the eyes.
If from the eyes should issue certain rays, by which the visual power is united with external objects, those rays are either corporeal or incorporeal. If corporeal, then when the eye sees stars and the sky, something corporeal issuing from the eye necessarily fills the entire space of the universe between the eye and the visible part of the sky, without diminution of the eye itself. But it is impossible that this should occur and also that it should occur so swiftly, the substance and size of the eye being preserved. If it should be said [instead] that the rays are incorporeal, then those rays do not perceive the visible object, since perception exists only in corporeal things. Therefore, the corporeal eye cannot perceive by the mediation of this insentient incorporeal ray. Nor do such incorporeal things return something to the eye, by which sight could perceive the visible object, since sight occurs only through contact between the eye and the visible form, because there is no action without contact. Therefore, if rays issuing from the eye return nothing to the eye, those rays do not produce sight. But if they return something to the eye, these are lights or colors, which appear by themselves [without the mediation of rays issuing from the eye] and which are multiplied to the eye among the [visual] rays. Therefore, [visual] rays do not cause sight to be applied to the things seen; but something else, which is multiplied to the eye, is by itself the cause of vision. It is therefore impossible for rays of themselves to cause vision, unless perhaps the lines drawn through the points of the forms multiplied from the surfaces of the visible object to the eye are called rays; for, as is evident by Theorem 2 of this book, in order that the object may actually appear, it must be possible to draw straight lines between any point on the surface of the visible object and a given point on the surface of sight. But such rays do not issue from the eyes. Therefore, the proposition is evident.
| Prepare a brief description of an authority in your
academic field of interest. This can be a person (living or dead),
a journal, a textbook, an institution....
Discussion points:
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