On Nature, Book 6 (De anima), Part 3, Chapters 1-5

8/14/2019 On Nature, Book 6 (De anima), Part 3, Chapters 1-5

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Avicenna

On Nature, Book 6 (De anima), Part 3: On Vision*

Translated by Erik Norvelle

Chapter I: On Light and Luminance

We must now speak of vision. In speaking of it, however, it will first be

necessary to speak of luminance and the luminous, of color and of the quality of

continuity which falls between the one sensing and the visible thing sensed. But first let

us speak about light. Since it is called luminance, light and a ray, it would appear

that in regards to the imposition of names, there is no great distance between these

three terms. Therefore it is necessary that, after stating these names, we

distinguish between them. But here there are three notions extending themselves, the

first of which corresponds to the quality which the vision apprehends in the sun and

in fire, and which is not distinguished as being white or black or red or any other color.

The second notion corresponds to that which shines brightly because of thesethings, i.e. the splendor which is seen to fall upon bodies, so that whiteness or

blackness or greenness is revealed in them. The third notion corresponds to that

which appears on bodies, as though it were spread out over them, hiding their color,

and which is like something which emanates from them; if this is in a body which

acquires it from another body, it will be called radiosity, whereas if it is in a body

which has it of itself, it will be called a ray. For now, however, we do not need the

terms radiosity or ray, but just the former two. But let one of them, i.e. that which has

this quality of itself, be termed light [lux], and let its effect be termed

luminance [lumen].

But that which we term light, i.e. that which the sun and moon have, is that which

is seen of itself. For the body which bears this qualitywhen there is something like airor water between it and the visionwill necessarily be seen, so that it does not need

anything further. Similarly there is that for which, in order that it be seen as it is in

itself, it does not suffice that there be air or water or something similar between itself

and the thing seen, but also requires that that which we call luminance first illuminate it

so that it can then be visible. This luminance will be in it as an affection of the light-

bearing body, provided that it be positioned opposite to that body and there be

between them a body which does not tend to hide the affection of the light-emitting

body and is receptive of luminance, such as air and water, which aid and do not impede.

But the bodies which are according to the first division are of two modes: i.e. those

bodies which do not tend to conceal others, as mentioned above, and which are

termed translucent, and those bodies which do tend to conceal, like walls and

mountains.

But ofthose bodies which do tend to conceal, one kind tends to be seen of

itself, so that it has no need of anotherto illuminate it, provided that there is a

translucent medium in front of it, and this is what we call luminous, like the sun

and fire; anotherkind is that which is not translucent, but rather hides that which is

behind it. You can see this with a candle when another candle is placed in front, since

* This translation is based on the Latin text contained inAvicenna.Liber de Anima seu Sextus de Naturalibus,parts I-III. Avicenna Latinus (Brill, Leiden, 1972). This translation is for informational purposes only, andshould not be cited for the purposes of academic publications without prior comparison with the Latin text.

Translation byErik Norvelle, published under a Creative Commons 2.0 Non-Commercial Share-Alikelicense.

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On Nature, Book 6, Part III: On Light and Vision Avicenna

one of these will prohibit the other from operating in the medium between them, and it

also impedes vision, so that the candle flame behind it cannot be seen. And one of

these kinds requires that there be there another thing present which causes it to have

a certain property, and this is something colored.But luminance is a quality of the first member of the division, according to its

nature, whereas color is a quality of the second member of the division, deriving from

its nature. For a wall does not permit a light source to illuminate what is behind it, nor

is a body which is colored in potency luminous of itself; for color does not occur in act

except as caused by luminance. For when luminance illuminates some body, either

whiteness or blackness or greenness or something else of this kind comes to be in act in

the body; if, however, it does not light up the body, it will just be dark black,

although it is colored in potencyif we want to call 'color in act' whiteness or

blackness or redness or greyness or something similar to these. For whiteness is not

whiteness nor redness redness, except insofar as we see it; but it does not become

visible unless it is illuminated.But you do not posit that the whiteness which is not in such a way as to be seen by

us, or redness or other things of this sort, have existence in act in bodies, such that

when dark air prevents us from seeing, the air itself is not darkened; for nothing is

darkened except that which was illuminated. Air does not, however, prevent the

apprehension of something illuminated, even if it there is no light source in it, nor does

it hide the color which is in act in something. Imagine that you are in a deep cave, with

air in it which is entirely of that property which you call darkened, and suppose that

there is a light source illuminating a body outside the cave, placed in air which you

state to be illuminated: you will see that body, and the dark air which is between you

and the thing itself will not impede your seeing it, but the air, according to you, in

either disposition is like nothing. Therefore darkness is nothing other than thedisposition for seeing nothing; that is, that the qualities which are in non-translucent

bodieswhile they are in shadow and not illuminatedare nevertheless visible in

potency, but are not seen, nor is the air seen. Therefore, it would seem that it would be

visible to you if you were to shut your eyes: because the darkness would be the same,

and because you see it as your disposition would be, if you were in dark air. But this is

not the case. For you do not see dark air if you close your eyes, nor is the darkness you

see something in your eyes, except just the fact that you do not see. And this is nothing

other than the privation of luminance from that which is normally illuminated, i.e. that

which is sometimes seen: for luminance is visible, and that in which there is luminance

is visible. But the translucent is not visible in any way; darkness, however, is in the

subject of luminance, and both are in a body which is not translucent. Therefore, the

body whose color tends to be seen, when it is not illuminated, will become dark, and

then color will most certainly not be in it in act. But that colors are said to be present

there, although hidden, is saying nothing: for air does not hide them, even if it is

dark, when colors are present in act.

But if a man were to call colors those diverse aptitudes which are in the bodies

which, when illuminated, makes one of them white and another red, this might be

so; it would come about, however, due to an equivocal use of names: for whiteness is

most certainly only of that to which it properly pertains to be seen, and this does not

have being when there is between you and the white thing something pervious which

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passes the thing seen, yet the whiteness itself is not seen. But the pervious thing

sometimes is pervious in act, sometimes in potency. In order that it be pervious in

potency, it need not be changed in itself, but ratherthere is change in another and

movement in another, and this is the quality of being permeable and transitable; andin order that this be in act, it does not need anything in itself, but is necessary for the

existence of that which penetrates and transits in act. Indeed, the permutation which the

pervious in potency requires in order to be pervious in act, is the permutation of the

colored body in order that it be illuminated and have its color in act; the motion, in fact,

is that the luminous body be moved with regards to [ad] that pervious body without

the permutation of that body (however, you already knew the certainty of this via the

premisses of this argument). But when one of these conditions is obtained, the

visible is transmitted and the pervious in potency is made pervious in act due to the

existence of something other than itself. We must resolve the truth about this

transmission, but we will put this issue off until later, when we enumerate the

problems which occur relating to the issue we been discussing. The solutions of theseissues will make it easier to affirm our own position.

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Chapter II: That a ray is not a body, and its differences from bodies enumerated.

Some thinkers state that the luminance which descends from somethingluminous onto bodies is not a quality which occurs in the bodies, but rather thatthere are extremely tiny bodies which separated from the luminous body on every side,which travel the distance from the luminous body, moved by the motion of thatbody, and when they fall upon other bodies, these are illuminated by them.Others, indeed, have stated that this luminance is nothing in itself, but merely themanifestation of that which is colored; Others, however, said that the luminance whichis in the sun is only the powerful manifestation of its color, which overcomes thevision.

But we must first attend to the truth in regards to these opinions, saying that it isimpossible that this luminance and this ray of the sun or of fire which falls upon

bodies, be bodies in which this sensed quality exists. For either they will be pervious,and then their perviousness must disappear when many are condensed to become onethinglike the tiny parts of a crystal are transparent, but that which is formed fromthem by condensation is not perviousor else they will be pervious things whose

perviousness is not lost, and then they will not be luminous (for we have alreadydistinguished between the luminous and pervious). But if they become non-pervious

because of thickening, their thickening will cover up that which is between them, andthe more thickening there is, the more concealment there will be. But in fact, the moreluminance is condensed (if it is something that can be condensed), the more it revealsthat which is visible; Similarly, if these illuminated things were initially illuminated,they would not be pervious, like fire and the similar things. Therefore it is clear that

the ray in colors is not a body. Again, it is impossible for a body to be moved naturallyto diverse places. And again, if those parts were bodies that are separated from aluminous thing and then impact the thing which is illuminated, then when thesphere of the heavens becomes dark, either these bodies would have to beannihilated or completely changed, or else a cloud will spread over everything. Butthat a cloud spreads over everything is unbelievable, because this is something whichoccurs suddenly. But annihilation because of darkening is of the same sort ofunbelievable explanation: for how do they assert that a body, when it penetrates

between two other bodies, will be annihilated by one of them? But permutation infact results in what we stated earlier, i.e. that the thing is illuminated by theopposition of a luminous thing; and when it is darkened, it is permuted. And if this is

the case, what necessity was there that these tiny bodies come straight from a part ofsomething luminous, and why do these bodies not permute by themselves because ofthe permutation of the luminous body?

But one argument which the authors of the theory of rays rely on is that which theystate as follows: a ray doubtless descends from the region of the sun and arrives herevia the region of fire, and this is motion; but no other motion exists except that of a

body. Again, a ray is moved by local motion, i.e. by the local motion of somethingluminous; but only bodies have local motion. Again, a ray strikes against somethingand rebounds from that to something else; but rebounding is, without a doubt, amotion of a body.

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But these arguments are false, and their propositions are all false. For in saying thatthe ray descends or is emitted or enters, we are using metaphorical terms,

because none of this actually occurs < ... >; for when it occurs from a height, it is said

to be descent, and this is in the manner of an accident, insofar as it appears thus fromits disposition; but this is not in fact descent: for it is not in any way seen to be intransit, nor does it require sensible time. And it is necessary that either a proofdemonstrate that it descends but how could they do this? or else that the sensesindicate those things upon which their argument rests but how will the sensesindicate the motion of a mobile thing whose time of transit was not sensed, and whichwas not sensed in the middle of its trajectory?

But the postulation of the local motion of the ray is not preferable to that ofpostulating the local motion of shadow, since it would have to be the case that ashadow is a body which is moved by local motion; but no motion of these things islocal motion, but simply destruction and renewal; for when a equal distance is

renewed, this also is renewed. But if someone were to foolishly insist, saying that ashadow is moved locally, it will then be necessary that it be moved locally by light, orelse that light will be moved locally prior to itself and after itself. But if it is moved

because of light, and occupies the place of another light, we would thus posit thatlight occupies the entire earth, and then light will not have local motion, which doesnot cover unless there is shadow, and then this argument concerning the motion ofshadow will be false, i.e. that light will be moved before the shadow just as the shadowwould be moved. Therefore, let us posit that the luminous body remains motionless,and stands still, because, when it exists, light will also stand still with it, and thisinduces us to posit that the motion of that which has a shadow will be the cause ofthe extermination of light. But it is possible that many of them might think that either

light is moved from diverse parts by an existing luminous body, and then someplace will be obscured, or else that, since light flees from shadow, it will spring backand retreat thither where it withdraws from the shadow. But all these ideas are fables,

because shadow does not exterminate light, and neither it nor light are bodies,although both have motion locally: for this comes to be from renovation, and not

because the thing itself moves locally. However, the resplendence of a ray is also anadopted word: for a shiny body, when illuminated, tends to be illuminated byanother body which is opposite to it, in such a way that it is not moved towards thatbody locally.

But the authors of the other opinionby which it appears that splendor does nothave being, but is supposed to be color itself as it appears in transparent and clearthingscan claim that, when splendor is observed in this medium, it is thatwhich appears with color, due to the accompanying colored clarity. But this clarityitself is not something in vision itself, but is a certain something which happens to thevision due to the comparison of that which is more lucid to that which is less lucid,and that which is of greater luminance is of a more evident color, because the lucidthing is of a stronger affection. For the luminance which is in a candle is only a littleless than that which is in the moon, but luminance which is hidden in the moon is onlya little less than the light which the sun casts by day in covered houses, or morecorrectly, in those places having a shadow where there is no ray of the sun: for thislight of the moon is destroyed in the shadows of houses, and, when the sun rises, is

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Chapter III: On what we must say about color

We say that, concerning the appearance of color, there are two opinions to understand

here: one is the form of color in act, the other is the appearance of color which has its per se

being in act in vision. But the first opinion indicates the accident of color and its being; the

second, on the other hand, indicates the relation of color or the being of the relation itself. But

the second of these is clearly false. For if it is posited that luminance itself is the relation of

color to vision, it must be the case that luminance is a relation or an accident of a relation,

and does not have in itself a stable being. But if they want to say that luminance will be

produced from a color of this sort, which, if it were seen, everything of this sort would then

be seen: either it will be the color itself, or else something else which co-occurs when

something extra is removed, just as when a darkening is removed, or similar things; but if it

were the color itself, this itself will be the first mode; but if it were a disposition which occurs

because of that which appears, then luminance will be because of color.But concerning the first opinion, it is necessary to say that the appearance is the outflow

of potency to act, and then the sun is not shining after this same single moment; or else they

want to say that the appearance itself is color, and then that which is said to be

appearance is nothing, but it will be necessary to say that luminance is color; or else they

want to say that there is a disposition adjoined to color, su that it would be color to which

luminance occurs sometimes and darkness at other times: color, however, in either disposition

will have its being in act. But if it were the very relation to that which appears to be present,

the second opinion will be the result; indeed, if there were another, that too would result in

the same thing.

But if we concede that light, even if it were itself color, is color which is in act, it

would then be necessary that either light be predicated of all color which is in act, or else thatonly white is a color, and that black be darkness, and it will be impossible that the color black

be illuminated by light. But this is not impossible, seeing that black can be illuminated and

can illuminate something else. Therefore, light is not just whiteness. But if light were not just

whiteness, but every color, then something which is light will be contrary to another which is

light; but light is only opposite to darkness; there that opinion is impossible. Further, the

theory that black is lucid, is contrary to its blackness without a doubt, and for this reason also

it is contrary to whiteness. But color, i.e. the nature of the genus of it which is in blackness, is

black, and the color which is in whiteness, is whiteness itself, not something which occurs

accidentally to it. Therefore pure generic color is not light itself.

Again, sometimes by light a translucent thing is manifested, such as water or a crystal;

but when they are in shadow and there falls upon them just light, it reveals them and theybecome transparent, and this is light, and not color. Again, there is something both luminous

and colored, from which sometimes only light shines on another thing, and appears in water

or on a wall, but sometimes, when it is stronger, there appears light simultaneously with

color, such that sometimes it causes the wall to become red, or the water in which it appears.

But if light were the detection of color and shadow were the occlusion of color, the effect of

the color red on that which is opposite would be redness, not just illumination. But if this

were the presentation of another color, why, when it is stronger, it produces in that which is

opposite to it the occlusion ofthat thing's color, and from this by a stronger color carries

the color to that thing, although the opinion of this man states that redness or greenness or

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other things of this sort are mixed together from the manifestations of whitenesses and the

occlusions of blacknesses? Therefore it follows from this that, when the color of some body

is made manifest by the ray which irradiates it, and from this there proceeds flashing, in

addition to the notion which is apprehended from it, the light of the other body has color, sothat its color does not appear in addition to that one.

Further, it is necessary that of this illuminated thing which illuminates another, the

manifest parts give color either by themselves or else with another. But if they do it by

themselves, then they themselves should only manifest the color which is in them, in order

that they may cause whiteness, but not hiding the color of it when they cause redness or

greenness. But if they are with the others, such that both those things having manifest color

and having disguised color would cause their effect, the former detection, and the latter

concealment, then the concealment of the color will have an effect in that which is opposite to

itself; but the concealment of a color would not have this disposition: for is it not the case that

you see that, when it is alone, that the concealment of color does not affect that which is

opposite it, as does the manifestation of color which they state affects when it is alone?But if they were to say that color is a manifestation, along with redness and greenness and

similar things which derive from redness and greenness, that redness or greenness, when they

are highly visible, operate as they themselves are and produce greenness or redness, we

respond: What kind of thing is thus such that, when it is only a little manifest, the color of

that which is opposite appears just as it is, and, since it is understood in terms ofthe theory

that it is just light alone, it is activated as much as the shining thing which does not have color

would be activated; but when the appearance of it is strong, it destroys that and conceals it

because of the other color by which it is covered? But it would firstly be the case that it

would not be activated in it unless it had a little of its own color, and afterwards, when it

increased, it would be activated in it more intensely, and so that whatever disposition it

produces, it would not exist except to hide the color of that thing and mix in its own color.But things are not this way, but first its color appears very evidently, and nothing appears in it

except the color which is in it by its aptitude, which, if something shining be nearby, would

notshine greenly or redly because of the disposition of it; afterwards indeed, when it

becomes more visible, it reverts to destroying its own color and to hiding and to covering

with another color which it does not have by nature. Therefore, one of these actions is from

another cause rather than from the other. And indeed the other comes from light: indeed a

body, if it were not to have color and were to have light, would do this just as a translucent

crystal does. The otheraction, indeed, is from its color when it is very manifest because of

this light, such that it rebounds in another. For we, even though we say that light is not the

manifestation of color, do not negate, nevertheless, that light is the cause of the manifestation

of color and the cause of it rebounding onto another.

We say, therefore, that light is a part of the composition of this visible thing which we call

color, and is something which, when mixed with a colored thing in potency, from both of

them there comes to be that which is color in act because of a mixing-together; but if there

did not exist this aptitude, there will be luminance and splendor by itself alone; for light is

like a part of that which is color and a mixing together with it, like whiteness and blackness

have mixtures from which there come to be the other middle colors.

But the statement of the one who said that light and luminance are nothing but the

manifestation of color, and that which he later says about those things which shine at night, is

refuted by the fact that a candle and the moon often suppress the luminance of those things

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and manifest the color of them; but it must be the case that the luminance of a candle be a

greater revealer of color, and it must be the case that, for that whose color is revealed because

of a candle, its color not appear in darkness. But this is not so, nor is it as they stated.

But that which was stated, i.e. that the sun and the stars have colors, and that lightobscures their colors, appears to be true, because some have colors in themselves, which,

when they shine brightly, their radiance is augmented so that it surpasses the vision and the

color is not sensed. Some indeed have light from color, and this is a thing for which light is a

natural companion, not coming from another source. Some, however, are mixed

substances, either by a mixture of composition from the parts of other luminous things and

other things having colors, or else by mixture of association from qualities, like Mars or

Saturn; but I cannot conclude anything concerning the sun now.

Therefore, we now know the disposition of light and the disposition of luminous things

and the disposition of colors and the disposition of translucent things. For light is the quality

which by its essence is the perfection of the translucent qua translucent, and is also somehow

the quality in it which is visible of its own essence and not by another, and without a doubtthat which is visible of its own essence prohibits the seeing of that which is beyond it.

luminance, indeed, is that quality which a body which is not translucent exchanges due to a

luminous body, and the translucent body is caused by that quality to be translucent in

act. But color is the quality which is perfected by light, and tends to put the body which

hinders the effect of the light source caused by that between which and the light source there

would be the medium itself. Therefore, bodies are shining and colored and pervious.

But there were many who said that certain bodies were visible because of a quality is in

their essence, and some others were visible because of an extraneous quality which he said

to be translucence. But the first member divides into two, of which one is that which appears

in the translucent thing essentially when there exists the trait of being something shining; < ...

> the second is that which needs, in order that it be visible, darkness and perviousness, justwith animals which shine at night by something which shines, such as the moon, or like

putrid scraps of oak and certain worms; indeed, I once saw a chicken's egg of this sort, and

dead locusts and dead crickets like this.

But this opinion is neither accepted nor true. For a shining thing is seen by itself both in

the dark and in the light: for if someone was present to see the luminous thing transmit

light, it would be seen; if, indeed, there were no-one there with it, nevertheless it would be

visible also, like the fire which a man sees in light, for there will either be its light or that of

another, and it will be seen in darkness. But the sun cannot be seen in darkness, because it is

located opposite to the vision of the one seeing; but when it rises, it fills the world with

light and does not permit any dark place. But the stars are not visible except in the dark, since

their luminance is much less than that of the sun, and thus they do not cast light on things or

illuminate them; nevertheless they do not prevent these things from being seen, and it

possible that, when there is darkness, that they be seen in the darkness, but not because

darkness is the cause ofthe stars themselves being seen per se. But it must be known that

among luminous things, there are some which surpass the others so much that they are not

seen, like the light of the sun which surpasses the weak light of fire and the light of the stars,

such that it is not seen to shine in the presence of the light of the sun, not because darkness is

necessary for them to be seen, but because it must be the case that of themselves they be

shining things that are not dark in relation to our vision. Indeed, when the sun comes to be

absent, they appear and are visible, because they are shining in relation to our vision because

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of the disposition which is in our eyesight. Perhaps, however, the judgment about fire and the

moon against a luminous thing which is weaker than them, this very thing is the judgment,

i.e. that it must be that the light itself is not a relation to us, when fire or the moon appears to

us, but there must be darkness, and thus they will appear, i.e. it is necessary that there not besomething there which overcomes their luminance, and thus they will be visible and vision

will be able to sense them. But you know that the dust particles which are in air are not of the

genus such that they, illuminated by other things, are not seen except in darkness, but, if a

man were in darkness, and the ray of the sun were to illuminate them, they will be able to be

seen. But if the man were in the ray, then by no means; and this comes about because of that

which is in < ... > by the light of the dust particles. For the vision of man, when it is

overcome by much light, will not see them; but indeed when it is not overcome, it will see

them. Similarly, those things which shine in the night are not of any other genus but that of

shining things, and they differ not in the universality of their nature, but in their

weakness; indeed, just as these differ from shining things in the generality of nature, and the

stars similarly.But this division is not true, unless it were said that there are some shining things which

overcome others, and some which are overcome by other. But the meaning of this is not that

one affect another, but rather is in our vision, just as there some hard things which are harder

and others which are less hard. Therefore it should not be said that those things which shine

at night are another species or genus per se from the genus of the colored or shining, but, that

they are of the universal nature of the shining things which are overcome by those which

exceed them in shining brightly, so that they not are then seen together with them because of

the weakness of our vision: For our vision, indeed, cannot sense them unless there is

eliminated the clarity of the refulgent things which overcome our vision. But if they support

this opinion, the division is good; but they do not support this opinion; moreover, they posit

that shining things are one genus, and colored things another, and this is different.

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Chapter IV: A consideration of the aforementioned theories about colors and theiraccidents

But concerning the issues we must unravel concerning these things, one is that we must

consider another opinion concerning color and light which, unless it is resolved, it will not bepossible to demonstrate the firmness of our opinion according the the division.

Therefore we say that of the opinions concerning color, one is that of those who hold that

the color white does not come about except by air and light or its brightness, and that the

color black is from their contrary. For the color white does not come about except by

translucence, when this itself is divided into extremely small parts which later are condensed:

for from this it occurs to it that its surfaces receive luminance and shine; and because they are

translucent, they transmit luminance from one to another; and because they are extremely

small, there comes to be a continuum of sorts out of them; and because the translucent is not

seen except by the color of an extraneous thing, therefor their perviousness is not seen, but

the reverberations of those condensed surfaces appear to be continuous, and the entirety

appears to be white. Therefore they state that because of this, the froth of water seems to bewhite, and snow likewise is white, since it is made of extremely tiny inanimate parts which

are translucent, among which air is mixed in, and light is diffused among them. Indeed,

crystals and heavy glass are not translucent, but no matter what continuous surfaces there

were of them, its perviousness would be destroyed; but, when any part of it is brought to exist

by itself, it will become translucent; indeed, in the translucent which is of a larger body, when

it is divided, the place where it is divided appears white.

And they also say that black causes a putative privation in the depth of the body and of

the luminance and perviousness simultaneously. But a certain thinker among them posited

that water is the cause of blackness, saying that, when this is moistened, it appears to have

some blackness; and they say that this comes to be because water expels air, and water does

not transmit luminance air, nor does luminance penetrate through it to the surface, and thusthe darkness remains. And a certain thinker wished to claim that blackness is the most solid

color, and the origin of colors, because it is not separated; therefore it is impossible to stain it;

whiteness, indeed, occurs in the translucent because of its density. But it is not a far distance

for the first opinion to result in this one: for it posits that the solidity of blackness does not

transmit luminance because of what does not transmit luminance, namely the solidity of the

color which reverberates from it.

But others stated that all elements are translucent and, when they are brought together,

there occurs from them whiteness just as we stated, also so that that which is opposite to the

vision be equal surfaces of the translucent body, in order that vision might penetrate them;

and that blackness occurs when the configuration of the body which is opposite to the

vision is strongly angled, so that it prohibits the transmission of luminance to the extreme

which touches the vision, then these angles, although they are illuminated, luminance does

not penetrate them sufficiently and therefore there comes to be darkness.

But from all this collection of opinions, that which is most difficult for me to judge

definitively, is that which states that whiteness comes to be because of luminance (lumen) andthat black is the true color. For we know that translucent things become white when dust is

ground up and mixed with air, like sesame powder which becomes white because the

retention of air in itself is mixed with the perviousness which is in its nature. And we also

know that black does not receive any other color in any way, as whiteness receives another

color in place of whiteness because of its own perviousness, and that it is a bare subject apt

to receive other colors

; because, however, it is denuded of qualities, it itself is receptive ofthem, so that it is not necessary for it to lose anything for this purpose; for that which is

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occupied by one, does not receive another unless the first is removed. And these are those

who posit that the origin of color is perviousness and non-perviousness.

But there are other thinkers contrary to these who neglect perviousness, saying that all

bodies are colored, and it is impossible that any body be lacking color, but there are manypores and empty spaces in bodies, whereby the rays which come from luminous bodies

penetrate through them to another part, and the rays of vision also penetrate them, and that

which is beyond them will become visible.

Indeed, we say that the first opinion is thusly, but not in continuous and conjoined bodies:

for color does not appear except in their density; for when they are conjoined and made wet,

whiteness is removed from it to that which is conjoined and dry. And indeed gypsum is not

white because of the grinding which reduces it into tiny parts, but because decoction makes it

such that, when it is wetted and later dried, it will be more strongly white because of the

combination which occurs to it. But that which this demonstrates to us is that, if the action of

fire does nothing to gypsum other than to facilitate grinding, then much facilitation of

grinding which produces extremely tiny parts would produce this same action in gypsum andcalcium and similar substances; and whatever grinds thusly to produce a power and later with

water is made into a paste, will become white like gypsum; but this is not the case. And

indeed effecting the grinding of something easily crushed sometimes produces the result

we have mentioned, but this is not the cause of whiteness coming to be.

Again, we posit that in gypsum this comes about just as we have said: but not all

whiteness comes about in this way. And indeed with eggs, when cooked, the clear white

becomes opaque and white, and it is impossible to say that fire will add rarefaction and

dispersal to it (for indeed it does not add anything to it in any way except constriction), nor

that air enters it and is mixed with it. And indeed when the egg is cooked it becomes heavier,

and this occurs because of the air which is separated from it. Secondly because, if there were

air there and it were mixed into its humor and whiteness were to be produced there, it wouldrequire a larger space.

Again, the medicine that quacks produce and call milk of virgins is made from oil in

which crystallized litharge is cooked until it is dissolved, and then is filtered until the oil

becomes translucent and white, and it is mixed with water in which old alum is mixed and

filtered many times until it is like tears; but if it is produced negligently, the complexion that

is desired will not be achieved. Indeed, when these two waters are mixed, the translucent

dissolved crystallized litharge coagulates, and is highly white, like sour milk, and then it is

dried. And this does not occur thusly because the translucent thing there is that to which a

division occurs: crystallized litharge, indeed, was dispersed and dissolved in the oil, and

neither were there present many translucent particles which, coming together, would be

bound fast; but it is necessary that the dispersion become greater because of the old alum-

water, nor also does a mixture occur with these things and the external air in any way; for this

does not occur except by some manner of permutation. Therefore, according to my opinion,

not all whitening occurs as these thinkers say.

But if whiteness is nothing other than light, and blackness is nothing other than that which

they say, then blackness and whiteness will not ever combine except in one manner. But the

proof of this is that white is converted into black in three ways. One is first in the slightly

grey, and this progression is pure: for if there were a pure progression, it would first arrive at

slightly grey, and then to grey, and then so long until black comes to be, because this,

proceeding in one way, does not cease by degrees reaching blackness alone, until pure

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blackness comes to be. Another way is from slightly red, to red, and then to black. The third

is from greenness, then to blue, and then to black. And in these modes there cannot be

diversity except from the diversity of that from which the middle colors are composed. For it

there were not there anything but white and black, and there were not in the origin ofwhiteness anything but light, then it will be impossible that they differ in this way, or that

they proceed in composition of whiteness and blackness except in just one way, in which

there will be no other diversity but according to more or less alone; and these diverse ways

would not exist.

For if there exist diverse ways, it will be necessary that there exist there some mixture of

something which is neither whiteness nor blackness, and it will be necessary that there be a

mixture of a visible thing. But among things there is nothing which is posited to be visible,

which is neither whiteness nor blackness nor a composite of these, except light, according to

the thinker who posited light to be something other than these: if his theory is refuted,

there will not be able to come to be a permutation of color in diverse modes. But if it this

permutation were possible, it will be necessary that there be there a third visible thing besideswhiteness and blackness. But this third visible thing cannot exist, unless we posit light to

exist as well as color. And according to this theory, it will be possible for colors to be

composed, and when white and black were mixed per se, there would be a procession along

the way of slight grayness. But when light is mixed with blackness and there was

something like a cloud over which reverberated < > like black smoke which is mixed

with fire, there will be redness if blackness were dominant, or there will be lemon-yellow if

the blackness is overcome and there were there present a superabundance of shining

whiteness; indeed, if the lemon-yellow were mixed with black and there was no brightness in

their parts, green would occur; and altogether when blackness is more hidden and whiteness

more manifest, redness will come to be from the converse. Therefore, if blackness were

dominant in the first case, a dark color will result; indeed if blackness were dominant in thesecond case, it would be of the color of leek leaves; but if it were greater and greater, there

will result an intense greenness that has no name. But if there is mixed in whiteness, there

will result the color of the wood lily; but if to this lily color there were mixed black with a

small amount of red, blue will result; but if to the red blue is mixed in, carmine will result.

And in this way colors can be composed, and the same goes for the mixing of colors or

composition of bodies but it is already known that blackness does not get transmitted

to other colors by reflection of the color black in any way, so that indeed it would be

necessary that from green colors or red colors nothing be reflected except whiteness, and

from the black parts nothing would result, especially when they are weak and worn down.

But if they say that these things would be seen to shine when mixed, the response to this will

be that from the mixture there comes to be an action and a passion and, by means of that

mixture, there come to be quality. And it makes no difference whether this comes to be by art

or nature, although nature can effect a combination which is in the manner of permutation,

while art can only produce mixture, and perhaps after this nature will produce some

permutation. And indeed nature can attenuate the combination which is in the mode of

mixture and grinding into minute parts, while art cannot achieve so much: and indeed there

are infinite actions of nature in dividing and composing in potency and act, while art cannot

elicit into act anything which is latent in those. Therefore it is now demonstrated that is it

most certain that whiteness in things is not light.

But we do not negate that air has some disposition to whitening, but not in the way that

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they state, except that it effects a combination which whitens. Similarly, we cannot say that

all the whiteness of sesame powder is as they state, but it is from combination: for air makes

a white color not only because of mixture, but also because of permutation. But if their

theory were solid, it would happen that a white or colored thing be slowly weakened until itsdensity were carried away, so that it would be translucent or slightly translucent. But this is

not so.

But that which they say, i.e. that black is not receptive of other colors, either they want to

say this from the concept of permutation or else from staining. But if they want to state this

on the basis of the concept of permutation, they are already in error: for what convinces them

is youth and the grayness of old age. But if they understand this in terms of staining, this is

from the disposition of closeness, not of a quality. But it is not far off that that which is black

not become black except when there is present a virtue of penetrability and retention and

constriction, i.e. so that it be mixed and penetrate and inhere; and that the whiteness which is

in things be different from that in their nature, and thus whiteness cannot invade blackness

and penetrate it and adhere to it, although also this is not impossible; for when they mix inceruse or in something similar via some way of penetrating and dissolving the blackness

which is in it, whiteness is produced.

But the second opinion is not easy to hold, unless we posit the existence of vacuum. And

indeed in regards to the pores which they mention, they must either be full of a body or else

empty. But if they are full of a body, the body itself will either be translucent because of the

pores which it has, or else it will not have pores (and this is different from what they say), or

else it will be terminated by vacuum and the postulation of the existence of vacuum will

necessarily result; but vacuum does not have existence. Therefore these say that not all pores

are apt for transmitting luminance unless their position is straight and without curves, so that

rays may be transmitted in a straight manner by them. Therefore let us make a white spear

either of crystal or of translucent white sapphire; let these pores which are in them bestraight and translucent, and let us posit that they are along the length, and the same along the

breadth, and thus they will either be transverse or from whatever part you want. Therefore

how will there be straight pores which are transverse to other straight pores, so that there will

not be tortuous curves from the part which you inspect? Therefore it necessarily will occur

that from some parts there is a difference of straightness, and there will exist parts which do

not have pores in the straight lines which they claim to go straight to the eye, or else the

entire body will be empty, and this is impossible Therefore it is necessary that when there are

diverse places in the translucent thing, their perviousness will vary for you altogether. Next,

how will there be a disposition of the body in which there are so many pores and emptiness

that its color is hidden, so that it will appear almost to be without color, when the thing itself

has in itself color, and it would not cover over the color of another thing which is behind it,

nay more, when it transmits that which is behind it? But if the thing itself hides its own color,

it will not do this except by that which is nearly nothing. Therefore the pores which are in it

will be much more than the plenum which is in it. But how will this construction of sapphire

come about, when the thing itself is nothing but vacuum? But if a man made in the sapphire

three or four holes and then squeezes with slight strength, he can break it. Therefore this

opinion is false.

Therefore, colors have being, and their being is not because they are light, nor is light

their appearance, although that which they are when in act does not occur without light;

for the translucent has being; and that is what we intended to demonstrate up to this point.

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But it remains next to say how the disposition of seeing comes about, on which also depends

the demonstration of how light is transmitted by the translucent.

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Chapter V: Concerning the diversity of opinions about vision, and the refutation ofincorrect opinion concerning the nature of vision itself

We say, therefore, that the most prestigious opinions on this issue are three in number,

although each one has subdivisions. One of them is the opinion of those who hold that radiallines are emitted from the pupil, in the form of a pyramid whose apex is against the eye and

whose base is against a part of the thing seen; and therefore vision is more truly said to be

like an arrow, i.e. the middle line of the radius, equidistant from the extremes, and to see

something is to move this arrow over it. But another opinion is of him who holds that the ray

exits from the pupil, but the totality of it not only reaches so far as to touch the hemisphere of

the heaven by means of the dispersal from which the propagation of vision comes about, but,

when it goes out and unites with illuminated air, the air becomes an instrument for vision,

which apprehends by means of this instrument. But the third opinion is that of him who

holds that, just as other sensibles are not sensed because some sense touches them or

penetrates them or combines with them or transmits information to them, thus vision does not

come about by a ray going out in some way and touching the thing seen, but rather the formof the thing seen comes to the vision, with the translucent medium carrying the form.

But the holders of the first two opinions argue saying that it is was not necessary for the

other senses that their sense objects come to them, except that the apprehension of them

would not be registered were it not for touching, as in the case of touch and taste, and like the

sense of smell, which gathers the odor to itself by attraction so that it will be affected by it,

and like sound, which movement in the air brings to the hearing organ. In vision, however,

this cannot be the case: for the thing seen is at a distance, and thus that which is directly

adjacent to the organ of sight cannot be seen, nor either can sight be impinged upon by

accidents which are in the visible body, such as its color and figure: for accidents do not

impinge; and since the thing is thusly, it must be the case that the power of seeing extends as

far as the place of the thing sensed, so that it might contact that thing. But the power ofsensing cannot be moved by local motion except via a body which carries it there. But this

subtle body is of the genus to which ray and spirit belong, and thus we call that it ray. And

because in the eye there is a body of this kind, it therefore seems to a man in darkness that the

light descends from his own and eye shines out from over his own nose or over anything else

which is near to that or opposite to it. Again, in the morning a man who awakens, when he

opens his eyes, appears to see rays before his own eyes. Again, the hole of the iris is filled up

by the light of the other eye when the other is closed; but when one stares intensely, then

there is no doubt that there is a body which is transferred to another. But the second school

of opinion denies that a body like the eye could grasp visual contents just by the ray

which continues in one line between the vision and the fixed stars, and still less by many lines

which disperse to all those things which are seen in the world, especially when it is equally

the case that anything seen by the ray would not be seen unless there were a continuum

uniformly extended. Therefore, it must be the case that because these things are in fact

seen, that there is a continuum. Further, they also deny that this emitted ray could move in a

time which is not sensed, from the eye all the way to the fixed stars. They say further that it

must be the case that the comparison of the motion from you to a thing two cubits away, with

the time of the motion which is all the way to the fixed stars, would be the comparison of two

intervals; therefore between the two times there would appear a difference.

Perhaps the writers of the third opinion argue this way against the authors of linear rays

because they do not know that this argument is frivolous. Indeed, they could posit a non-

sensed time which is extremely brief, during which the ray is moved all the way to the fixedstars. Therefore, this time can be divided infinitely, and they can add a part of this, or a part of

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a part, which would be related to the entirety just as a brief interval is related to a long one,

although either of the two times or the two intervals would not be sensed, because they are

extremely brief. But the authors postulating rays have a reason which is difficult to refute.

That is that they say that mirrors give evidence for the existence of rays and their radiance.Indeed, it is necessary that the form of the mirror return to the vision after the form of the

opposed thing seen has returned to the mirror, or else that it be as we say: and indeed the ray

issues out and impacts the mirror, and then it is reflected so that it impacts that from which it

is transmitted according to some given angle. But although the first argument was refuted,

there remains the second.

But because the refutation of the first opinion has already been given, i.e. that these

forms, if they were formed as images in the mirror, without a doubt these images would be in

some location of the surface, just as when color and light are transmitted simultaneously by

the translucent medium outside of the first subject, and, when that transmission of

radiance is viewed from one and the same place, it appears differently according to the

diversity of the sites looked at. But the image which is in the mirror is not of this sort; on thecontrary, they are moved locally because of the local motion of the one looking at them.

For if they were not moved locally except just by the local motion of the thing seen, there

would not be this difficulty. But that which is moved by local motion of the one viewing,

means that the true location is not in the place where the form is imprinted, but, when the

viewer moves, the direction of the line also moves which, when it is transmitted to the thing

seen, produces a proper angle, and the visible things are seen by this same line, and by the

same line also another part of the mirror appears in which the same visible things appear,

and thus it does not cease moving.

Also they also stated that a proof of the rightness of this position is that in the vision of

a man a simulacrum of visible things is sometimes impressed and transmitted thence to the

sight of another viewer, such that the second viewer would see this, but it is not seen by the possessor of the pupil, in whom this simulacrum is assimilated to the image of the

imagination. But if this impression were truly in his organ of vision, it would have to be

the case, according to the authors holding the theory of simulacra, that both would equally

apprehend this simulacrum. And indeed the same thinkers hold that the truth about the one

seeing is that the simulacrum of the thing seen appears in his visual organ; thus in

whosesoever visual organ a simulacrum appears, he must see that thing.

And they also say that it ought to be the case that upon looking at a mirror, one should be

able to see oneself seeing one's own form, but it is not so. And indeed the ray, when it impacts

the mirror and apprehends it, it rebounds and is transmitted, and impacts upon the form of the

observer and apprehends him, and because he sees the mirror and himself via the straightness

of the outgoing radial line, it appears to him that the one is in the other.

And they also say that this means that the form of the thing seen is not impressed in

the mirror, that is, when they see it, there is no doubt that it is not on the surface of the mirror,

but in its depth, and as if it were at a distance from it. And this distance must exist, or else

the form of the thing seen would be in the depth of the mirror: but the mirror does not have

that much depth, nor also, were it to have that much depth, would it transmit to us that which

is imaged in it; it therefore remains that this distance be, conversely, a distance from a part of

the depth of the mirror, and it is true that the distance itself is not apprehended except in

that distance which is between that thing seen and the mirror, because its image is not

impressed in the mirror.

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But we must first refute the two first opinions, and affirm the truth of the third, which is

ours; thus, we return to this opposition and resolve it. Therefore we state first that that which

goes out from the visual power, must either be some stable being which has place and is a

bodily substance, or else it must be something which does not have existence of itself, onlyhaving being due to a translucent thing which is between the organ of sight and the visible

thing; but that which is of this sort should truly not be said to be emitted by the organ of sight,

but it should be said that it is a passion of the air due to vision, and that the air, because of its

being affected in this way, becomes a tool for seeing: but this comes about in two ways, either

by the aid of the medium, or by the aid of an instrument.

But before we pursue this division, I will make a general judgement, that sight does not

come about by a change in air because a disposition by which it aids in sight in any way. For

this disposition would doubtless come to be an affection in the air, not an intention which is

relative to one viewer and not to another. But we do not negate that this division exists, but

we say that there must be a relation between the air and the viewer when he sees, and because

of this relation vision comes about; we deny, however, that a fixed and stable disposition andaffection exists in the air itself and in its essence, such that air would come to have any

quality or property in itself, although it is not permanent nor exists once the cause is removed.

And indeed air does not have this kind of affection in comparison with one visual power and

not another, but it is forthe visual power in comparison of all that is, just as the white

thing is not white in relation to one and not to another, but is white in itself, and is white in

relation to all that is, although it does not remain white, once the cause of the whitening has

been removed. Thus it must be the case that this affection either receives intensification and

abatement, i.e. so that sometimes it can be stronger and other times weaker, or else it must

always be in the same way. But if it were always in one way, its cause must receive more

or less, or not. But if the nature of the cause were to receive intensification and lessening, and

that nature were the cause by itself, that which is caused would necessarily follow it inreceiving intensification and diminishment. But it is impossible that it receive remission or

strengthening, if its strengthening and weakening were something in its own nature according

to which it is the cause. Therefore it follows from this that when there are many agent virtues

in the air, the accidents of this disposition and this arrangement will be in the air more

strongly, and that the virtues of vision and the disposition of the air in this arrangement will

reduce the weakness of vision, especially when the white is not present because it does not

receive intensification and weakening; indeed, vision would be from the mode of the

virtues and the dispositions of the virtue. But its virtue is not, as we have said, in the relation

to one seeing power and not another, but it is of itself. It therefore follows that, those who

have weak vision, when they come together, would see more strongly, and, if they were

separated, they would see more weakly, and the person of weak sight, when he was right at

the side of one who sees more strongly, would see better, if the air were more strongly

permuted because of that affection, in whatever manner it was, or else from the conjunction

of many or strong causes, because that which transmits form and assists vision is stronger,

although the weakness of vision would also cause a defect in this. And indeed the gathering

of many people with bad vision at the same time is not like the situation when there is just

one person with bad vision, to the extent that for the person with bad vision there does not

exist an equal disposition of his vision in clear air and in thickened air: and indeed for the

person with bad vision, when aided by some extrinsic tool, his action will no doubt be

stronger; but we see that the person with weak vision is not helped even a little bit in his

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vision by the presence of a person with better vision, nor from the gathering of many people

with weak vision at the same time. Therefore, the antecedent is false.

But now we return to the division which we posited earlier, saying that it must be the case

that air is either a medium, or else an instrument. But if it is an instrument, it will either be asensing instrument or else a transmitting instrument. But someone cannot say that air is

changed to become sentient, so that it would sense the stars and transmit to the vision what it

senses. For it follows that air does not touch everything that is seen, for the fixed stars are

visible, which air does not reach. But it is absurd to say that the stars, which are in the

middle, are affected by our vision and become an instrument for it, as airis said to be its

instrument: for this theory receives no support from prudent thinkers. Or else we

might say that light is a body dispersed in the air and in the heavens, united with our visual

powers, such that it becomes their instrument. But even if we concede this absurd point, it

must still be the case that we do not see the whole body of a star, and we will have to

concede another absurdity, i.e. that in the heavens there are pores which cannot be equalized

at the midpoint of the heavenly body; further, it must be the case that no matter what star islooked at, only one part of it will be seen, and not the whole. It would indeed be a powerful

virtue of our visual powers, if they were able to change the entirety of the air and light which

is diffused in the bodies of the heavens, as these thinkers themselves state, and the whole

were to become a perceiving power, or something else of the sort. Next, air and light are not

just connected to one person's vision and not to others. Why, therefore, would those things

which they sense transmit to one person's vision and not to others? But if it is necessary

that a person's vision be in opposition to the thing seen in order to see it, such that the air

transmits to it that which it senses, then the sensing of air is not the cause by which the

sensible things themselves are transmitted to the soul, but the person's vision must be

located in some opposite point to the thing seen, and between them the air is a medium. But if

air senses by itself and also transmits, what does it matter for us if it senses of itself? Becausewhat is seen is what the air brings to our senses, and we do not care whetherthe air senses

of itself or not. But if they want to say that its sensing is our sensing, then the heavens and air

and all things sense because of us.

If, indeed, air were not proposed as being an instrument, but rather as a medium which

is affected first by the thing seen, and later perfected so as to be a medium, we must ask

which passions it is affected by in order that it transmit: evidently, if it is posited that air

receives the virtue of life from the vision, while itself is a simple element, this is impossible;

otherwise, we must posit that it becomes pervious in act because of vision; however, the sun

is more effective in making it pervious in act, and thus more sufficient. Would that it were

known what it is that vision causes in this air! For if it heats it, it must be the case that when

air is cooled, sight will be prevented. But if it cools the air, it must be the case that when air is

heated, sight will be prevented. Similarly, in regards to the remaining contraries: for every

contrary into which air can be permuted has causes other than vision which, if they were to

hold, it would not be necessary that the air be permuted because of vision, just as, if their

contraries were to hold, it would not be necessary that it be permuted by vision. And perhaps

it is not because of vision that perviousness occurs, nor does it occur because of a quality

which has some contrary to that which is known, but rather it occurs because of a property

which has no name: therefore, how will these authors of this opinion know this, and how will

they comprehend it? But we have already related the common theory, which prohibits all

these permutations, and the same holds if they are compared to a property which has a

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natural name. Therefore we posit that, when air is translucent in act and colors are colors in

act < ... >, it is not necessary that there be anything else in order that there be vision.

But let us posit now that that which goes out from the eye be a substantial radial body, as

many of these authors wish. Therefore we say that its disposition requires that there bethese four modes: i.e. that it either be in contact with the whole thing seen and that it not be

separated from the one seeing, or that it be in contact with the whole thing seen and that it be

separated from the one seeing, or else that it be in contact with one part of the thing seen and

not with another, whatever be its disposition with the one seeing, whether separated from the

thing seen and from the one seeing.

But the first option is most false, for evidently there is a continuous body which goes out

from the eye and contacts one hemisphere of the world and touches the celestial bodies;

therefore, when one eye is closed and the other is opened afterwards, a different body will go

out than the first; or else, when the eye is closed, the whole collected body will return

to the eye, so that, when it is opened again, it will all go out from it, as though this were due

to the choice of the one closing the eye. Therefore, why is someone at a distance not seenin his figure and size, if vision reaches to him and touches him? For indeed magnitude should

be more perfectly sensed than color: for perhaps the ray disperses and becomes more rarified,

and color appears like a confused color, but the dimension is seen like an uncertain

dimension; but the uncertainty of the dimension does not at all take away its entire

magnitude, and, because the visual ray has become dispersed, it is as though it were

composed of corporal dimension and nothing, or of a non-body. And in this case the angle

which is in the vision does not carry to us, but it carries according to the holders of the

simulacra theory, who say that the simulacrum occurring in a mirror appears in the part of the

postulated pyramid belonging to the crystalline surface, the head of which is within it. For if

the angle were larger, because the thing is closer, that part of it will be larger and the images

which appear in it will be larger; but if the angle were smaller, because the thing is furtheraway, the part will be lesser and the images which appear in it will be smaller; But according

to the theory of those who posit that the thing seen is that which is touched by the instrument

of vision, this angle does not carry to them.

Indeed, the second option is more clearly impossible, for it is evident that, if that which

goes out from the eye is separated from the one seeing and reaches all the way to Ursus

Minor and touches it, while there is nothing continuous existing between the constellation

and the one seeing, and the one seeing senses that which he himself sees, it will be like saying

that someone can touch with a detached hand, or else that which the shorn tail of a snake

touches is transmitted to the serpents own body, unless it were said that that which goes out

permutes the medium, and by its very self it transmits something to the vision, and then the

air will be both the transmitter and the permuted, simultaneously. But we have already spoken

sufficiently on this issue.

But if it were continuous to it from any part of the thing seen, it would be necessary that

the whole not be seen, but only that which it touches. But if it is posited that the thing seen

were changed into the nature of the outgoing ray and they were like a single thing, what do

we say of the heaven when we see it? Will we be able to say that the heaven is changed into

the nature of that outgoing ray and that so that it becomes sentient due to that ray, so that

the two are as a single thing, and so that the ray contacts the star of Saturn and sees the

entirety of it, and similarly with Jupiter and the other large stars? But the falsehood of this is

clear. Next, we have already shown that this permutation of air cannot exist, in the

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arguments that we have given. But if it were said that translucent air is not united with

vision to form a single thing with it, but ratherthe air is changed into a transmitting

nature, and it is that which the ray touches < ... >, and because it does not touch, the air

transmits its form due to the permutation which occurs to it, the first response to thistheory is: why is the air not permuted because of just the pupil and transmits to it, if it

were indeed to transmit, so that it would not need the outgoing body? The second, indeed, is

that we have already declared the impossibility of these permutations. The third opinion is

that the mediating air, which is between the two outgoing lines, must transmit every one of

those things which it transmits to the other, and finally, by the collection of rays from the

collection of air entering between the lines, the form of the thing sensed is transmitted twice

or many times, and then the thing sensed must be seen twice or often, especially if the

opinion of another of these thinkers were to hold, i.e. that the lines are not sensed per se, but

only that which the air transmits due to them. Therefore, if the pupil receives transmission

due to both of these, i.e. the lines and the air simultaneously, then the air is transmitting

simulacra, as the first author stated. But he who knows that in these things there is no vacuumand that the bodies of the heavens are solid and there is no emptiness nor concavity in them,

will also know that this theory is impossible and that whatever goes out from the eye

cannot penetrate these bodies. But how can this ray which goes out penetrate water, unless

there were vacuum in it, so that it touches the entire earth which is beneath the water and sees

it, when the same earth is continuous with the water and the water's mass does not grow

because of that which is mixed with it? But if there were vacuum there, how much will be the

magnitude of these empty vacuums which are in the water, even though water is heavy and

descends into empty spaces and fills them? Perhaps it seems to you that the entirety of

water is empty spaces, either the larger part of it or half, so that this outgoing ray can

penetrate to the entirety of the thing which is beneath the water and contact it and touch it,

with this ray not being separate from vision: for if it were separated, it would be anamazing thing.

But if someone were to say: We see that small things penetrate much water and color it,

just as saffron does, of which a little colors much of the water, we say that the coloration of

much water by a little saffron must come about in two ways: i.e. that either the tainting of the

water would not occur except in the parts where there is saffron, and those parts of the water

will be changed in themselves because of the taint, just as they are changed because of

hotness and coldness and odor, not because the substance of the other is mixed into it. And

this permutation comes about either according to the permutation of a true staining, or

according to the permutation of a similitude to staining: I say that it is a similitude, just as

when images of something lying in it the water appear on its surface and not opposite to the

vision, or else as when waterappears to be of the same color as its container; but if there

were much of this, the surface of the water will appear according to that taint, although there

is little of the dye in it. But if this tainting occurred in the second way, this comparison will

not be useful for this theory: for indeed the water is permuted and transformed; for a little of

the saffron will have penetrated into the whole of the water, because sometimes something of

a greater mass is changed by that which is of great strength but small mass. And in general, if

air were of a disposition such that it is changed by rays, we would incur in the position

which we negated before, i.e. that it must be that when the rays are numerous, many

permutations of the air will come about, and the presence of many will aid in seeing. But if

it occurred in the manner of transmission without permutation, then the nature of air is such

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that it transmits the simulacra to the recipients, and so it would also transmit to their

vision.

But if it were not in the manner of the second member of the division, but of the first,

then we cannot doubt that a minimum of water is mixed with a minimum of saffron [red dye],and conversely, since a minimum of water is without a doubt of a greater quantity than a

minimum of saffron, and that between any two minima of saffron there would be pure water,

and that these minima of pure water which are between any two minima of saffron are

much greater than the minimal parts of saffron, such that a comparison of minima to minima,

when they were each collected together, would be like the comparison of a whole with a

whole. And if it were thusly, the minimum quantities of saffron would be extremely small and

it would be impossible for them to diffuse amongst the entirety of the water, and then they

would not be able to touch the water totally. But this explanation is false. For the entirety of

the water is seen to be tainted, and this comes about from one of two causes. It is either the

case that any minima of saffron and water are so small that they are not distinctly

apprehended by the senses, but this does not mean that some are not much greater thanothers: for bodies are divisible infinitely, and it is possible that a minimum of water be 1000

times larger than a minimum of saffron, and nevertheless is of such small size that each

minimum cannot be sensed separately. And howsoever it may be, vision will not be able to

discern between the minima of red dye and water, but from them both there will appear to

be one diffuse coloration in between red and transparent; and this is one possible

explanation. Or else, the sensible minima of saffron will not be in opposed and equidistant

places, but when two minima of saffron are ordered such so as to be above one sensible

minimum of water, the other minima of saffron will similarly be below, so that, if they were

elevated with the others, they would cover the surface of the water, and there will be some of

them which are seen in the higher surface, and some of them would transmit their simulacra

to the higher surface, and all of them coinciding in simulacra according to one color, thewater would indeed transmit the color of any one of them because of its transparency, and it

would appear to be a continuous whole in one surface, and the saffron would appear to be

diffused or spread out through all the water. But this is not the case. But the evidence for this

theory is the small amount of the tainted thing seen in something sparse which has no density,

and the large quantity of it which is seen in something deep, although the comparison is

similar. And indeed the comparison of saffron in a sparse thing to the sparse thing itself, is

like the comparison of the saffron which is in a deep thing to the deep thing, and by these two

ways that which is small can overcome or taint that which is large. But certainly something

small does not overpower that which is great in quantity, but perhaps does overpower in

apparent quality.

But if they were to propose that this small thing which goes out penetrates in the air and

does not arrive all the way to the thing seen, and later the distant air transmits to it, and the

air itself transmits to the vision, either because the air transmits to it because of its own

transparency, even without permutation, why therefore does the air itself not transmit to

the pupil, making unnecessary the labor of sending out a spirit into the air, which has been

shown to be a nuisance? Indeed, if it were because of permutation, we have already said what

is necessary on this issue. Then why does the air not permute the pupil, so that there is no

need for a spiritual body?

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Documents

On Nature, Book 6 (De anima), Part 3, Chapters 1-5