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Philosophical Perspectives, 21, Philosophy of Mind, 2007

WHAT CHANGE BLINDNESS TEACHES ABOUT

CONSCIOUSNESS

Fred Dretske

Duke University

Change blindness is often described as a failure of subjects with normal

vision to see the visible objects and/or properties whose presence or absence

constitutes a difference (change) in sequentially observed scenes. The evidence

used to support this charge of blindness is the failure of subjects to notice or

detect the difference these objects and properties make.

If, though, one doesnt have to notice a difference in order to be conscious of

the objects and properties making up that difference, the striking deficits revealed

by change blindness tell one absolutely nothing about what a person is conscious

of. They reveal nothing about perceptual experience. They reveal something,

perhaps, about what a person knows or doesnt know, what she thinks or doesnt

think, but nothing about what she sees or fails to see.

This critical assumptionthat noticing or detecting differences is necessary

for awareness of the things constituting the differenceis an assumption that

many participants in this research would happily admit to making. More often

than not it is simply taken for granted. For many psychologists and philosophers,

conscious perception of objects just is perception of them (usually understood

as the receipt of information about them) with an associated awareness that

one perceives them. In philosophy the idea is expressed by HOT (higher-order-

thought), a theory of consciousness according to which conscious experience is

simply experience one knows and, therefore, thinks one is having.1 In psychology

the assumption is called the subjective test for awareness, a widely accepted

operational criterion that identifies conscious experience with experience the

subject knows she is having and is, therefore, able to report having. 2

Despite its widespread acceptance, though, this assumption is false. It is

false in a way that seriously distorts the character of perceptual experience.

It represents conscious experience as informationally more impoverishedand,

therefore, less rich and texturedthan it actually is. Ridding oneself of this

assumption, then, is an important step in getting a clearer, a more realistic,

account of perceptual experience.

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What Change Blindness Teaches / 217

a difference, while still failing to realize that there is (or that he constitutes) a

difference in the group of people she saw on the two occasions.

This, it seems to me, demonstrates quite conclusively the irrelevance of

change blindness to conscious perception of the objects (and properties) consti-tuting the difference. Change blindness is a cognitive, not a visual, impairment.

It is more appropriately described as change ignorance.5 One can be blind to

visible differences and still see everything (i.e., every object and/or property) that

people who notice the difference see.6

It would, after all, be exceedingly strange if Sarah, on her second observa-

tion, were only aware of the same seven people she saw the first time. Why does

she only fail to see, why is she only blind to, Sam? Why not one or more of

the other seven people? Or, despite Sarahs protests (she thinks she saw, maybe,

a half dozen people), isnt she aware of anyone at the table? If Sarah is, as shesays, aware of some people around the table, does her visual system somehow

know that Sam is a new, an additional, a different member of the group and, as

a result, create a localized blindnessa scotomathat prevents her perception

of just him?

That sounds absurd. It seems more plausible to suppose that on the second

viewing Sarah saw Sam in the same way she saw every other person around the

table, the same way she saw all seven people the first timeviz., consciously.

Her visual experience the second time was different from what it was the first

time, and the experience, on both occasions, was conscious. Sarah simply fails tonotice, fails to detect (therefore, remains ignorant of and, hence, fails to report

on) a difference that she consciously experiences. One can be conscious of objects

that constitute a visible difference and not be conscious of the fact that one is

conscious of them. This is why we cannot use change blindness to conclude

anything about what subjects are conscious ofespecially not when dealing with

complex, multi-element stimuli.

You are looking for a friend in a crowded marketplace. You cant find him.

Later, after youve found him, he tells you he was standing directly in front of

the fruit stand you looked at several times. You, in effect, plead blindness: Ididnt see you. Wrong! You probably did see him. You just didnt recognize him.

He was, after all, standing in your line of sight only a few yards away in broad

daylight. You certainly didnt see through him. You didnt, for instance, see the

apples on the stand directly behind him. The reason you didnt see the apples is

because he was in front of them, blocking your view of them. So you must have

seen him. The reason you didnt recognize him is that you mistakenly thought

he wore his red sweater today so you, without pausing to study faces, scanned

by the blues, browns and grays you saw in the crowd.

Im not arguing that a person is always conscious of all visible elements ina complex display. Only that one can be, and often is, aware of more than one

realizes. If Sarah looks at seven hundred people in a room, seven thousand in a

parade, or seventy thousand in a soccer stadium, all clearly visible from where

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she is sitting, does she see them all? I dont know. She probably doesnt see all

of them. She certainly sees a lot of them. The subjective impression of having

seen hundreds, even thousands, of distinct elements in such conditions (I readily

concede to Dan Dennett 1991) may be an illusion. It may be an illusion. Whetherit is an illusion seems to me to be an empirical matter (I return to this point in

3). It depends on how much and what kind of information one actually gets

about these individual objects, and it is important to remember here that not

much information about an object is needed to see it. Information about color

isnt necessary or we wouldnt be able to see things at dusk, in dimly lit rooms, or

in peripheral vision. Information about shape isnt necessary. Things look pretty

much the same shape at 600 yards or (in the case of stars) 1,000 light years.

That doesnt prevent their being seen. If you get enough informationand this

isnt muchfrom x to enable you to wonder of or about x What is that?, youperceive x with awareness. It is conscious perception. A question about whether

Sarah is conscious of Sam, then, could be put this way: when Sarah sees the

group the second time, could she (not did she, but could she) have wondered

Who is that? about Sam? She can wonder this without ever realizing he wasnt

there the first time.

If this is the correct way to interpret the results of change blindnessat least

some cases of change blindnessthen the first lesson to be learned from change

blindness is that noticing or detecting x is not necessary for being conscious of

x. You can be conscious of x and not think you are. You can be conscious ofthings that make a difference and think you are not.

This, though, leaves us with a troubling question. If you dont have to know

you are aware of something to be aware of it, if you dont have to notice or

detect it to be conscious of it, what, then, makes perception conscious? If Sarah

isnt the authority on whom or what she is conscious of, who is? If it isnt her

thinking she sees him that makes her perception of Sam conscious, what does?

This brings us to a second lesson of change blindness.

2. Seeing and Knowing

Though one can see an object without realizing it, without knowing anything

about it (this is the first lesson), awareness of it is constituted by an experience

that enables on to have direct knowledge of it.

Think, once again, about Sarahs second observation. Although no one

captures Sarahs attention, any member of this group, including Sam, could

attract her attention. I couldnt attract her attention, at least not while she is

looking at the group around the table, because I am standing behind her. Shecant see me. The picture on the wall behind Sam could not attract her attention

because she doesnt see it; Sam is in front of it. But Sam, if he had been wearing

a clown costume, if he had been standing on his head, or if he had been stark

naked, would have been noticed. He would have attracted her attention. The

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same is true of the other seven people. The fact that Sarah would have noticed

him if he had been naked or standing on his head suggests7 that Sarah is getting

sensory information about Samthe information that he has clothes on and

is standing upright. She gets the same information about every member of thegroup. She didnt, perhaps, pay enough attention to tell you what anyone was

wearing, but that everyone was wearing something or other was clear enough.

Sarah, therefore, can tell you something about Samnot, it is true, about Sam

by name (she doesnt know who Sam is), not, it is true, about Sam as the new or

a different member of the group (this is the fact she is blind toi.e., ignorant

of), but, nonetheless, about Sam as one of the people she saw around that table.

We can even imagine her asked to testify about Sams behavior at a wild party.

She was an eyewitness. Her testimony goes as follows: if, as you tell me, Sam was

standing at that table the second time I looked, and if, as you assure me, no onewas blocking my view of him, I can assure you he was not naked. None of them

were. I could see that much.

Under normal circumstances and with respect to objects we are attentive

to, visual experience supports, it grounds, it justifies, a host of beliefs we have

about the things we see. It gives us knowledge of them. The man Im talking

to, Sam, is standing upright. He is not standing on his head. How do I know

this? I can see that it is so. What makes my experience of the person conscious

is that information embodied in this experience is the basis, the justification, for

my current conscious and expressible beliefs about the person.But experiences can carry such information without generating any beliefs

at the time they occur. This is what happens in Sarahs case. At the time Sarah

sees Sam, she doesnt think to herself: that guy is wearing clothes, he is standing

upright, he isnt dressed like a clown. She doesnt think anything at all about

Sam. She doesnt even notice him. Yet she has, in her experience of him, a great

deal of information about him, information she can later exploit in telling us

something about the men she saw around the table. Her experience of Sam is

conscious for the same reason my experience of Sam is conscious: the experience

(in my case) justifies and (in Sarahs case) it is capable of justifying a consciousjudgment about the objects seen. The only difference between Sarah and me

is that Sarahs judgments were deferred. A judgment only occurred later, when

she was asked about the people around the table. My judgments, on the other

hand, were cotemporaneous with the experience on which they were based. Both

experiences carried the information that made this knowledge possible.

Perceptual knowledge of the world is sometimes of this delayed sort, the sort

that Sarah has of Sam when she later thinks about the people she saw earlier. This

is the kind of perceptual knowledge I might have (or acquire) when someone asks

me about whether there were any giraffes in my bedroom this morning. I knowthere werent because I could see there werent. I didnt have giraffe thoughts at

the time, of course, but now that I think about it, my current beliefs about my

room qualify as perceptual knowledge on the basis of the experience I had this

morning.

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This sort of delayed perceptual knowledge is such a pervasive phenomenon

that we often, charitably, give ourselves knowledge at the time at which the

experience occurred despite the absence (at the time) of any belief or judgment.

I knew this morning that there werent any giraffes in my room despite notthinking about it until now. And one can hear Sarah insisting that she knew, at

the time she saw them, that none of the people around that table were wearing

clown suits. She didnt think about it until now, it is true, but she has, from her

experience of the group, all she needed to know. She didnt, she will insist, just

find this out. She knew it all along despite not thinking about it until asked.

This suggests an epistemic test for awareness, a knowledge test for what

a subject is conscious of. The rough idea (refinements in a moment) is that if

you can see (and thereby know) that x is F for some value of F, you must be

aware of x. You cant know, by seeing, that x is F without being conscious ofx. Even if Sarah didnt notice Sam, even if she paid no attention to him, if she

could see that he was (or wasnt) F for some value of F, then she must have

seen him. So if Sarah now knowsor could knowbased on her past visual

experience of the group, that none of the people at that table were naked,8 then

since Sam was one of the people at that table, Sarah knows things about Sam.

So she must have seen him. The fact that she didnt, at the time, see that this

was so (she had no thoughts at all about this subject) is irrelevant. The fact that,

without any change in her experience, she could have known it and the fact that

now a judgment based on that experience qualifies as knowledge shows that herconscious experience carried this kind of information about Sam.

Two qualifications are important. (1) You can, of course, see that x is Fthat

Sam has clothes on, for instancenot by the way x (Sam) looks, but by the way

something else looks. Seeing a photograph of x, the indications of an appropriate

measuring instrument, or a prearranged signal from a knowledgeable accomplice

will do. So it must be understood in this test that the perceptual knowledge of

x must be (what I will call) direct. The knowledge of x, that it is F, must be the

result of the way x itself looks, not some information-carrying intermediary. (2)

The perceptual knowledge in question must be of xs possession of some propertyor feature that does not affect visibility. We do not want to infer, for instance,

that S sees (from ten yards away) a flea on Fido just because S can tell (see), from

the general appearance of things, that none of the fleas on Fido are the size of a

basketball. Size affects visibility. In these conditions and at this distance you can

see things the size of a basketball but not things the size of a flea. So the fact that

you can see (hence, know) that nothing on Fido is the size of a basketball, does

not mean you can see x just because x is a flea on Fido. You can, perhaps, know

(by vision) things about the fleas on Fido (that none of them are the size of a

basketball), but from this it does not follow that you see the fleas on Fido. Color,shape, orientation, and dress, however, are not like that. These properties affect

an objects noticeability (how much attention they attract), but not its visibility.

Being naked in a crowd of clothed people (or, for that matter, being clothed in a

crowd of naked people) affects how much attention you will attract, how easily

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you will be noticed, but it does not make you (as opposed to the parts of your

body) visible. A red square in a crowd of green squares will be the one you notice

first. It will stand out in a very conspicuous way. But its color doesnt make

it visible. The green squares are just as visible. This is why I can prevent youfrom noticing (and, therefore, detecting or finding me) by wearing camouflage,

but camouflage doesnt make me invisible.9

So, given these qualifications, the suggested epistemic test for awareness is

this: S is consciously aware of x if and only if there are Fs (that do not affect xs

visibility) that S can directly see (hence, know) that x possesses. You are conscious

of x if your experience gives you (or, without change in the experience, could give

you) direct knowledge of x where the direct means that the knowledge is the

result of the way x itself (and not, say, a photograph or a measuring instrument)

looks.It is important to understand that this test is not a simple informational

test. It is not just information about x that is critical for awareness of x since

one can get information about x without ever being conscious of x. It is, rather,

the epistemological role of this information, its role in grounding a conscious

judgment (belief) that is important. Remove this epistemic dimension, as you

do, for example, in the case of perception of an object without awareness of it

(e.g., blindsight), and what remains may (given a liberal enough interpretation

of what perception is) qualify as perception of that object, but it will not be

conscious perception. A person who is not conscious of Sam might receivevisually transmitted information about Sam. The information may even causally

influence the judgments the person makes about Sam. Facts about Sam may, in

this way, play a part in the explanation, the reasons why, the person says and does

what she does (e.g., gets so many correct answers in a forced-choice situation).

But without consciousness these facts about Sam will not be a subjects reason,

her justification, for whatever judgments she makes about Sam. She will not be

able to see (and in that way know) that Sam is standing upright. Reactions to

Sam, if they occur at all, will not add up to knowledge. They will be guesses

correct guesses, perhaps, if they are reliably caused by information being receivedabout Sambut guesses nonetheless.10 Sarahs judgments about Sam, however,

are not guesses. She knows he wasnt standing on his head. No guessing about it.

She knows it because she could see he wasnt. That is what conscious perception

of Sam provides.

3. Operational Criteria for Consciousness

I expect to be told that if this is what change blindness teaches about

consciousness, change blindness doesnt teach us anything very useful. It certainlydoesnt give us a way of telling whether Sarah was aware of Sam. We are told

that Sarah is aware of Sam if she knows (or could know) things about Sam by

the way he looks. But how do we tell whether Sarah knows things about Sam?

How do we tell whether Sam is one of the people she sees to be standing upright?

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Sarah would believe, at least she would say, exactly the same thingthat all the

people at the table are standing uprightif, because of localized blindness (or

because someone was standing in front of Sam), she wasnt aware of Sam. So

even if Sarah knows something about Sam, she doesnt know she does. It is as ifshe didnt. It is as if Sam wasnt even there. So, from an operational standpoint,

from the standpoint of finding out whether Sarah was aware of Sam, it is of no

help to be told that Sarah is aware of Sam if she knows, by seeing, that assorted

things are true of him. Not if we cant tell whether she knows things about

Sam.

This is a fair comment and it raises a genuine issue that I want to confront

in this final section. As long as there is no way of telling whether Sarah is aware

of Sam, no behavioral test or criterion for showing that she is conscious of him,

there will remain a suspicion that such questions are really bogus. What is thescientific point, the purpose, in worrying about whether Sarah is conscious of

Sam if such awareness need make absolutely no difference to Sarahs reactions

to or dispositions toward Sam? One doesnt have to be logical positivist to want

facts about a persons conscious experience to give rise to something observable

(to others). Without that, it seems, there cannot be a science of consciousness.

I think what we have already learned about conscious experience, especially

the idea of it as an epistemologically enabling condition, can help us here. To

illustrate the way it helps, I use a more dramatic example of change blindness

than Sarahs perception of Sam. I use a wall containing several hundred bricksas an example of a multi-element stimulus. The question I ask is whether normal

subjects during brief observations (2 or 3 seconds), despite an inability to see

differences, are nonetheless aware of the bricks that constitute the difference

and, if they are, how this might be demonstrated. The answer I give and hope to

illustrate by use of the epistemological test for awareness is the same as I reached

in the case of Sarah: namely, that many, perhaps most, subjects, despite a failure

to see differences, are nonetheless aware of all the bricks in the wall and, in

particular, the brick(s) that make the difference. They know, or their experience

of the wall enables them to know, something about every brick.Look, then, at Figure 1, a brick wall with a few bricks missing.

When you look again (Figure 2) the wall has changed.

One of the missing bricks has been replaced. Do you see a difference?

Probably not. Seeing a difference in the number of bricks is seeing that there

is a difference, and I assume most of you didnt see this fact. We are not now

interested, though, in what facts you saw. We are interested in what bricks you

saw. Did any of you see, were any of you conscious of, Sam, (identified in

Figure 3) the added brick in Figure 2?

Did Sam make a difference in your conscious experience of the wall? Wasyour conscious experience of the wall different when you looked at Figure 2?

Since you didnt see the difference between Figures 1 and 2, I dont expect

you to know the answer to this question. But (for those of you with normal

vision) I do.

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Figure 1. A brick wall (with some missing bricks)

Look at Figure 4. This is how the wall would have looked to you if Sam had

been black instead of grey. Look at Figure 5: that is how the wall would have

looked to you if Sam had been tilted instead of horizontal. You know the wall

didnt look these ways when you saw it the second time. You would have noticedit if it had. You would have spotted Sam immediately. He would have popped

out. But you didnt notice Sam. So he wasnt either black or tilted. Thinking

back on your experience of Figure 2, you know that none of the bricks in the

wall were black or tilted. Since Sam was one of the bricks in the wall, you know

that Sam was neither black nor tilted. So, according to our proposed test for

awareness which tells us that you are aware of an object if your experience enables

you to know things about it by the way it looks, if you can (directly) see that it

is F for some value of F (that does not affect its visibility), you were conscious

not only of Sam but of every brick in the wall. When you viewed Figure 2 yourexperience of it was rich enough in information to give you knowledge of each

brick in the wallthat it was neither black nor tilted.11

Of course you didnt knowand perhaps you are still not convincedthat

you had this perceptual knowledge of Sam during your second observation

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Figure 2. The same brick wall (with one missing brick replaced)

(Figure 2). If you in fact came to know this of Sam, and this knowledge was

based on the way Sam looked to you, then you may be ready to concede: you

were aware of Sam. But you may not yet be convinced that you came to know

this of Sam in particular. Maybe, you will say, you came to know this of somebricks, and came to know it by the way these bricks looked, but how can you

be sure you came to know this of Samnot to mention each and every brick?

If you cant be sure of this, then, even if you accept the epistemological test for

awareness, you cant be sure you were conscious of Sam.

I understand and sympathize with such agnosticism. Nonetheless, Figures 4

and 5 are meant to convince you that during the second observation your

experience of the bricks was such that you could know (see) that Sam was

neither black nor tilted. You knew this, I claim, because you knew that none of

the bricks in that wall were black or tilted,12 and since Sam was one of thosebricks, you knew this of Sam. It was, in part, the way Sam looked to you that

told you that none of the bricks were black or tilted. If some other object had

occluded Sam, if, therefore, Sam did not look some way to you, you would be

unable to tell whether none of the bricks were black or tilted. So it is the way Sam

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Figure 3. The added brick (SAM) identified

looks that is essential for your knowing what you do about the wall. Figures 4

and 5, I submit, show that this is a reasonable conclusion about the particular

brick we are calling Sam. Since nothing, except location, distinguishes Sam fromother bricks in the wall, it is a reasonable conclusion about every other brick

in the wall. If there is doubt about this, it could be tested. We can show the

wall, again and again, controlling as much as possible for time of observation

and direction(s) of gaze in order to see whether you could tell whether any brick

was black or tilted. If results are the sameif, after brief (a few seconds13 )

observations, you can tell whether any brick is black or tiltedwe can conclude

that such brief observations give you knowledge (or, if not knowledge, then the

kind of perceptual experience needed for knowledge) about each and every brick

in the wall. Since it is the way individual bricks look that gives one this knowledge(cover any one of the bricks and you destroy a persons ability to tell whether

any of the bricks are black or tilted), you saw them all.

Let me, then, summarize, the argument that observers of normal vision

most of them anywayare (in observations of a few seconds) aware of every

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Figure 4. How figure 2 would have looked if SAM had been black

brick in the wall despite not being able to see differences in the number of bricks

they see.

First Step: Observers of normal vision will see some bricks when looking

at Figures 1 and 2. When you looked at the brick wall, you were conscious ofindividual bricks in the wall. You dont know how many you saw, and you dont

know whether there were the same number of bricks in Figure 1 and Figure 2,

but observers of normal vision will be quite sure they see individual bricks both

times. Their experience of the wall is not, in other words, like seeing a flock of

geese or a herd of cows at a great distance where the collection is seen (it looks

like a spot in the distance) but individual members of the collection are not

discriminated. I assume that normal observers experience of the wall in Figures

1 and 2 is not like that. They will not just see a brick wall in the sense of a

collection (pile, heap) of bricks. They will see individual bricks in the wall. If Iam wrong about this, if vision is such that one could not, at this distance and

illumination, see individual bricks when looking at these figures, then it seems

clear that such a person did not seetherefore, was not conscious ofSam in

Figure 2. I mean to exclude from consideration such people. I do not consider

them observers of normal vision.

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Figure 5. How figure 2 would have looked if SAM had been tilted

Second Step: In looking at Figure 2, you (those who saw individual bricks)

could see (and, hence, could in this way know) that none of the bricks were black

or tilted. Your conscious experience of the wall carried this kind of information

about the bricks.At the time you saw them you acquired, or could have acquired, a piece

of perceptual knowledgethat none of the bricks were either black or tilted. If

you arent sure your conscious experience of the wall justifies such a knowledge

claim (that it actually carried this information) and are, therefore, reluctant to

say you knew (or could now know) that (in viewing Figure 2) none of the bricks

were black or tilted, this could be tested. Does the collection of bricks you saw

look different to you if any brick is black or tilted? Figures 4 and 5 are meant

to convince you that the collection of bricks would have looked different if Sam

was black or tilted. We would have to see if this is true of each of the otherbricks. Perhaps, although you saw many bricks when looking at the wall, you

didnt see brick #247. Something (a speck in your eye) obscured it. Or perhaps

#247 was seen only peripherally (unlikely with the numerous saccades occurring

in the few seconds you viewed Figure 2) and information about its color (black

or grey) was lacking. If so, you didnt get the information that it was grey like

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the other bricks. Or perhaps #247 projected to the blindspot in your left eye and

you (contrary to instructions) viewed Figure 2 with only your left eye for only 50

milliseconds (during which no eye movements occurred). So you didnt actually

get information about #247.So whether or not your experience, during your observation of Figure 2,

carried information about each and every brickthat it was neither black nor

tiltedis an empirical matter. Maybe it did, maybe it didnt. I assume, however,

that normal observers are observers who view these figures in accordance with

instructions: for several seconds with both eyes at reasonably close range. The

claim is that these people will know that none of the bricks were black or tilted.

I assume this is most people.

It is, furthermore, the way individual bricks look to normal observers that

gives them this information. Cover or conceal one or more of the bricks and thekind of reliability we take to be characteristic of knowledge vanishes. One can

no longer see whether none of the bricks are black or tilted. So normal observers

have direct perceptual knowledge of each and every brick.

Step 3: So normal observersthose who I describe in steps 1 and 2when

looking at Figure 2 were conscious of all the bricks in the wall.

This conclusion is reached by applying the proposed criterion of awareness:

if you can see that x is neither black nor tilted, and this knowledge is direct (it

is the way x looks that carries this information), you must be aware of x. The

perceptual knowledge is grounded in a conscious experience of the objects oneacquires knowledge about. That, I conclude, is what perceptual consciousness

provides. It gives us knowledge of the world around us.

Notes

I am grateful for the buffeting this paper received at places I read early versions:

Delaware, Istanbul (Bogozici University), Connecticut, the ASSC-10 meetings in

Oxford, and Wayne State University. I would particularly like to thank Chris

Boorse at Delaware and (at Connecticut) Gunnar Bjornsson (from Goteborg

University), Paul Bloomfield, and Crawford Elder for saying things that prompted

me to make changes.

1. Rosenthal 1986, 1990, 1991 is an early and articulate exponent; also see

Carruthers 1989, 2000; Dennett 1978; Lycan 1987, 1992; and Armstrong

1968.

2. See Cheesman and Merikle 1984, 1986; Kanwisher 2001; Dehaene 2001; Dienes

and Perner 1996; Merikle, Smilek, and Eastwood 2001; Underwood 1996; Potter

1999; Dixon 1981.

3. To foveate an object is to direct the eyes so that light from the object is directed

onto the fovea, an area about 1 mm square on the retina densely packed with

photoreceptor cells. Vision is most acute when the image of an object falls on this

part of the retina.

4. Dretske (1999). This is a distinction I earlier described (in Dretske 1969) as non-

epistemic vs. epistemic perception.

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5. Wolfe (1999) describes change blindness not as a form of blindness, but as a

form of amnesia. Subjects perceive things, they just dont remember well enough

to report the perceived differences. It should go without saying that I have no

objection to an interpretation of change blindness as a cognitive phenomenon, afailure to know, notice, remember, understand or realize. Often an investigators

interpretation of the experiments is clouded by use of words like detect to

describe what subjects fail to do when confronted with differences. Is a subjects

failure to detect a change a failure to see it or just a failure to notice it (i.e., to

know that it is occurring)? If the latter is what is meant, why call it blindness?

6. I assume here that factsincluding the fact that there is a differenceare neither

visible nor invisible. They certainly dont emit or reflect light.

7. It suggests this, but it doesnt demonstrate it. The fact that you would notice x, an

object in peripheral vision, if it moved, for instance, doesnt show that you saw x

when it wasnt moving. Maybe its movement causes you to redirect your gaze sothat you now see it when you didnt see it before. I return to this point later.

8. This is de re knowledge of each and every member of the group. Sarah can know

of Sam that he is (isnt) F without knowing who it isviz., Samthat she knows

this about. This is why Sarah can know of (or about) the added person at the

table, the person ( = Sam) who makes a difference, that he is standing upright

without knowing that there is an added person at the table or that he makes a

difference.

9. This leads to some results that are superficially counterintuitive, but, on deeper

reflection, these results are, I believe, perfectly acceptable. For example, if you

paste a white sheet of paper on a matching white wall (perfect camouflage) sothat people cannot distinguish the paper from its surroundings (cannot, that is,

see where it is) do they nonetheless still see the paper? Yes. They certainly see

something in that part of the wall. It isnt the wall behind the paper. What else is

left?

10. I am thinking here, of course, of forced choices in blindsight studies and stem

completion choices in priming studies of implicit (unconscious) perception.

11. Incidentally, my claim that one is conscious of individual bricks in the wall

perhaps, even, of all the bricks in the wallis perfectly consistent with the claim

that there is no conscious perception without attention. In their careful study of

inattentional blindness Mack and Rock (1998) do not require attention to each

and every object consciously perceived. As long as there is what they describe

as distributed attention (Chapter 7, especially pp. 163-65) or objects fall in a

zone of attention (see Chapter 4) their attentional demands on consciousness

are satisfied. They count as attention to the individual objects in a multi-element

array attentive viewing of the array itself. Assuming that you looked attentively

at Figures 1 and 2 for a few seconds, then, you satisfied their demands on

consciousness of each and every brick in the wall even though you paid no

particular attention to any of them.

12. Once again (see footnote 8) it is important to understand the de re character of this

putative knowledge. In saying that one knows that none of the bricks are black

or tilted, I am saying one has 350 distinct pieces of knowledge about individual

bricks in the wall, not (necessarily) a single piece of knowledge expressible as

None of the bricks in the wall are either black or tilted. One might know

something general in nature, that none of the bricks are black or tilted, but one

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230 / Fred Dretske

neednt know this to know, of each one of the bricks, that it is neither black nor

tilted. One can have these singular pieces of knowledge about individual bricks

and because of ignorance or uncertainty about whether one sees all the bricks in

the wall (maybe some bricks are concealed) not have knowledge of the generaltruth that none of the bricks in the wall are black or tilted. So when I say that

you know that none of the bricks in the wall are black or tilted, I should be

understood as saying that for all x, if x is a brick in the wall, you know x to be

neither black nor tilted. I am not claiming that you know that for all x, if x is a

brick in the wall, x is neither black nor tilted.

13. A few seconds is enough time for six or seven saccades (involuntary movements

of the eye) so it is likely that one foveates most, if not all, the bricks in such brief

observations.

References

Armstrong, D. 1968. A Materialist Theory of the Mind. London; Routledge.

Carruthers, P. 1989. Brute experience. Journal of Philosophy, 86, 2589.

Carruthers, P. 2000. Phenomenal Consciousness. Cambridge University Press.

Cheesman, J. and Merikle, P. 1984. Priming with and without awareness. Perception and

Psychophysics, 36, 38795.

Cheesman, J. and Merikle, P. 1986. Distinguishing conscious from unconscious perceptual

processes. Canadian Journal of Psychology, 40, 34367.

Dehaene, S. ed. 2001. The Cognitive Neuroscience of Consciousness. Cambridge, MA; MIT press(A Bradford Book).

Dennett, D. 1978. Toward a cognitive theory of consciousness. In C. Savage, ed., Minnesota

Studies in the Philosophy of Science, 9. Minneapolis, MN; University of Minneosta

Press.

Dennett, D. 1991. Consciousness Explained. Boston; Little Brown and Company.

Dienes, Z. and Perner, J. 1996. Implicit knowledge in people and connectionist networks. In G.

Underwood (ed.), Implicit Cognition. Oxford University Press, pp. 22755.

Dixon, N. F. 1981. Preconscious Processing. New York; John Wiley and Sons.

Dretske, F. 1969. Seeing and Knowing. Chicago; University of Chicago Press.

Dretske, F. 1999. The Minds Awareness of Itself. Philosophical Studies, 95, 122.

Kanwisher, N. 2001. Neural events and perceptual awareness. In Dehaene, ed., 2001, pp. 89113.Lycan, W. 1987. Consciousness. Cambridge, MA: MIT Press (A Bradford Book).

Lycan, W. 1992. Uncertain materialism and Lockean introspection. Behavioral and Brain

Sciences, 15.2, 21617.

Mack, A. & Rock, I. 1998. Inattentional Blindness. Cambridge, MA; MIT Press.

Merikle, P., Smilek, D., and Eastwood, J. D. 2001. Perception without awareness. In Dehaene,

ed., 2001, pp. 11534.

Potter, M. C. 1999. Understanding sentences and scenes: the role of conceptual short term

memory. In V. Coltheart (ed.), Fleeting Memories. Cambridge, MA: MIT Press, pp. 13

46.

Rosenthal, D. 1986. Two concepts of consciousness. Philosophical Studies, 94.3, 32959.

Rosenthal, D. 1990. A theory of consciousness. Report No. 40, Research Group on Mind andBrain. ZiF, University of Bielefeld, Bielefeld, Germany.

Rosenthal, D. 1991. The independence of consciousness and sensory quality. In E. Villanueva

(ed.), Consciousness. Atascadero, CA; Ridgeview Publishing Co., pp. 1536.

Wolfe, J. M. 1999. Inattentional amnesia. In V. Coltheart (ed.), Fleeting Memories. Cambridge,

MA; MIT Press, pp. 7194.

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