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When a real object causes an image of itself to form on each of the retinas of

When a real object causes an image of itself to form on each of the retinas of our eyes, the image is upside down. It used to be thought that there was a 180 degree twist in the optic nerve to turn it right side up again, but then it was found that there is no such twist. So now it is believed that the image is reoriented in the unconscious mind. Does it not follow that when we see something, we see a twice inverted image of the real object, not the real object itself?

Questions along this line have been asked a few times before: See, for example, 987 and 988. The answer is that, no, it does not follow: You see the object. That there is an inverted image of the object on your retina is part of how you see the object. You do not see that image. I could see that image, if I looked in your eye, and I suppose you could see it, too, if you looked in a mirror or something. But with what precisely would you see it in the ordinary course of events?

The idea that the retinal image has to be "re-oriented" is really quite puzzling and probably a product of the same kind of mistake. To think the image needs to be reoriented is, it seems to me, to suppose that the spatial properties of the representation must be spatial properties of what is represented. There is simply no reason to assume that. If you turn a map sitting on the table around so I can see it, thus changing the spatial properties of the representation, the map does not suddenly represent everything as having been turned 180 degress. It continues to represent precisely what it did before. Similarly, that the image on the retina is, qua representation, inverted wtih respect to what it represents does not imply that it represents everything as being upside down. And if there is something like an image in the visual cortex—of course, it would not be an image in the straightforward sense, since it would not be something one could see—there is no reason it needs to be spatially oriented the same way as what it represents. It too might be upside down, or sideways, or whatever, and that would be quite irrelevant to what it represents.

Are sensations real? That is, do they continue to exist when unperceived? It

Are sensations real? That is, do they continue to exist when unperceived? It seems to me that objects that I perceive around me are both real (because outside my head) and composed of sensations: that is, they are structures of colours, tactile qualities, etc.; in which case these sensatations, as parts of real objects, are real. But it also seems obvious that sensations exist only while perceived, in which case they are not real.

Sensations are real in my book while I am having them, but you are right that it is not easy to use them to build a table. The trouble is that the table has a continuous existence, while it is only intermittently observed. One standard way around this problem is to fill in the apparently unobserved periods by having God observe the table all the time. That was George Berkeley's suggestion. Another way is to fill in the gaps between actual table sensations with merely possible sensation. This too is suggested by Berkeley in one remarkable passage, and figures centrally in John Stuart Mill's phenomenalism, acccoring to which tables are, in his memorable phrase, 'permanent possibilities of sensation'. The idea is that even when nobody is looking at the table, it is still true that if someone were to come into the office, they would see the table.

Phenomenalism is a clever solution to the gap problem, but like many other philosophers (not to mention ordinary people) I find it incredible to suppose that objects are built out of sensations, gappy or continuous, actual or potential. Sensations are in our heads, and sometimes caused by physical objects that are outside our heads and are not even partially made up of sensations. But the contrary, idealist perspective has certainly figured prominently in the history of philosophy.

It is generally agreed that perception involves a real object transferring

It is generally agreed that perception involves a real object transferring information about itself into the brain of the perceiver, via the sense organs and nerves; and the distinguishing features of this are that the real object is external to the perceiver and public, while the image of it in the brain is internal and private. My question is: illusions are unreal, but they are external and public --- as with the railroad lines meeting in the distance, or the Sun and the Moon being the same size during an eclipse. So are illusions real, or unreal?

There are really two different kinds of "illusions" one might have in mind, and they are "real" in different senses.

Consider first the railroad tracks. We can describe this phenomenon in purely geometric terms. Take a point P and a line segment AB. Then as AB is moved further away from P the angle <APB becomes smaller, and as the distance tends to infinity, the angle <APB tends to zero. So what we're dealing with is just a fact of geometry.

On the other hand, consider the horizon illusion: When the moon is close to the horizon, it looks much larger than when it is overhead. My understanding is that it still isn't well understood why, but whatever the reason, it has something to do with how our visual system works. So this illusion is real not in a geometrical sense but in a broadly psychological one.

If I cross my eyes, or press on one eyeball, I see double. This is explained by

If I cross my eyes, or press on one eyeball, I see double. This is explained by saying that we have two optical images, one on each retina, and they are normally coincident, whereas they are not coincident when pressing on one eyeball. I am now crossing my eyes and I see two computer monitors. Which is the real monitor: the one on the left, or the one on the right?

This question is obviously similar to this one. And the answer is much the same. You are not seeing two computer monitors. You are seeing one monitor, although it seems to you as if you are seeing two monitors. There is one monitor that both appears to be slightly to the left and appears to be slightly to the right. Note that this is not the same as to say that there is one monitor that appears both to be slightly to the right and to be slightly to the left. I doubt we could ever have a visual experience correctly described this latter way: Could it ever look to you as if there was one thing that was in two places? But there is no obstacle to our having a visual experience correctly described the former way: It can be that, although it looks to you as if there are two things (in different places, of course), in fact you are seeing only one.

If that seems confusing, well, it is confusing. The logic of this kind of situation is extremely subtle. Philosophers and logicians spent a lot of time in the 1960s trying to understand it, and they made a good deal of progress, but I doubt anyone would really claim fully to understand such constructions (that is, to understand quantification into opaque contexts).

That said, it may well be that, when you cross your eyes and such, you have two images of monitors in your head. But images of monitors are not monitors any more than drawings of monitors are monitors, so neither of them is the "real" monitor, although both of them are images of the real monitor. And again, you don't see these images. You see the monitor. Perhaps you see the monitor in virute of having the images in your head, but that is different.

A spoon half-immersed in a glass of water appears bent at the surface of the

A spoon half-immersed in a glass of water appears bent at the surface of the water. We know that this is due to refraction of light, which bends the rays of light at the surface, so that the retinal image of the spoon is illusorily bent. So we can speak of the real spoon, which is not bent, and the image spoon, which is bent. They have to be two, because one thing cannot be bent and not bent at once. Since the spoon that I see is bent, it must be the image spoon, not the real spoon. So where is the real spoon?

To my mind, the mistake occurs here the moment you start speaking of "the image spoon". There is no "image spoon". There is just a spoon, and it is in a glass, and you see it. (So the real spoon is in the glass, right where you thought it was.) The spoon looks to be bent, certainly, so perhaps it follows that there is an image of a bent spoon in your head. (Some philosophers would deny that, but I don't think we have to deny it, or should deny it, in order to resolve this puzzle.) But the image of a spoon is not a spoon, and it is not bent, any more than, if I draw a picture of a bent spoon, the graphite somehow becomes a spoon or becomes bent. It's just a picture of a bent spoon, made out of graphite on paper. Nor, most philosophers would hold, is the image of the spoon what you see. What you see is the spoon, in the glass. Perhaps you see the spoon in the glass in virtue of the fact that you have an image of a spoon in your head, but that is a different matter.

Please don't think I'm saying this is a silly mistake. It's not a silly mistake. Philosophers have made this mistake for a very, very long time, and I'm sure there are still philosophers who are inclined to make it. I think it's an important fact about the nature of human perception that we are so tempted to speak the way you do. But—one might well add this to the oft-requested list of subjects on which we philosophers have made some, however halting, progress—we have learned over the last few decades how not to make this mistake.

Can we perceive relations? For example, if I have a cup of coffee I can perceive

Can we perceive relations? For example, if I have a cup of coffee I can perceive the cup as white, round, hard, and shiny; and the coffee as liquid, brown, hot, and delicious; but the relation in has no color or visual size or shape, and I cannot touch it, hear it, smell or taste it --- so how can I perceive it? It's tempting to say that I cannot perceive it because it isn't real --- but if it isn't real then how could I drink the coffee? The similarity between two oranges, the direction of a train whistle, the relative brightness of the sun and the full moon ... There are countless empirical relations that can/cannot be perceived. How come?

I'd suggest that this puzzle is largely a linguistic one. Consider the relation being larger than. Can one perceive that relation? There's a temptation to say that one cannot perceive the relation itself, because the relation itself "has no color or visual size or shape", and so on and so forth. And maybe that's so. Ask a metaphysician. (Of course, what answer you get will depend upon which metaphysician you ask!)

But the examples with which you began suggest a different question. Can one perceive that one thing is larger than another? Here, it seems to me, the answer is clearly that one can. We perceive that kind of thing all the time. But how can we perceive the relation if we can't perceive the relation itself? The answer, I think, is that this question is just confused. What we perceive is that the objects are so related. Perception, as people sometimes put it, has propositional content, and relations figure in these contents. One might yet wonder how it is that we manage to perceive such things as that the coffee is in the cup. That, I take it, is a question for a psychologist more than for a philosopher, but certainly part of the story is that you can perceive where the coffee is and you can perceive where the cup is, and on that basis your brain might come to have a view about the relation between the coffee and the cup, which view is delivered to you in perceptual experience.

Perhaps someone else here would be able to recommend a place to start if you were interested in the empirical literature on such issues. There has, for example, been quite a lot of empirical work on the perception of causal relations.

When you see the Moon, which is about 250,000 miles away, does your

When you see the Moon, which is about 250,000 miles away, does your consciousness extend out of your head, for a distance of 250,000 miles, to the Moon, or do you see an image of the Moon, brought to you by reflected sunlight? If you see an image of it then you do not see the real Moon, while if you see the real Moon then your consciousness somehow has to get out of your head to that distance. So do you see the real Moon, or not? The real Moon and the image cannot be one and the same, because the Moon is made of rock, and the image is not made of rock.

Some philosophers do think that our consciousness 'extends out ofour heads' when we perceive things in our environment. But even theywouldn't hold that your consciousness embraces the moon now (ieas it is when you are perceiving it). For everybody must agree that youwon't see things that happen on the moon until after adelay of more than a second.

You also ask whether we see an image of the moon, ratherthan the moon itself. However, scarcely any philosophers nowadays wantto say that we see images, rather than physical things--this idearaises more problems than it solves. (Even philosophers who reject theidea that consciousness depends on how things are 'outside our heads'will generally hold that our perceptions represent physical objectsrather than images.)

The general consensus then, both amongthose who think that consciousness 'extends out of our heads' and thosewho don't, is that we see the moon itself, not an image, but the moonas it was more than a second ago.

You might want to argue that, if at some time t we see something, we must be seeing it as it isat time t. But why assume this? After all, we are generally happy toaccept that when we see things, we see them as they are at a spatialdistance. So why can't we see them as they are at a temporal distance?

Is the "theory" of the Matrix, or something along those lines, possible?

Is the "theory" of the Matrix, or something along those lines, possible? We perceive the world with little signals sent to our brain, so couldn't those signals he rigged to, say, a machine? And everything happening around us is just in our heads? If you dissagree with this, what could you use to prove me wrong? ~Kris S.

I do not disagree with this, nor (I suspect) would most philosophers. The story of The Matrix is possible.But as long as we're talking about possibility, your situation might beeven worse than the one depicted in the movie. At least in the filmwe're all sharing the same, collective hallucination, but it might bethe case that you're the only one plugged into the Matrix.Perhaps you don't even have a full body; you might be just a brainfloating in a vat of nutrients and connected to a computer that isfeeding it electrochemical signals.

In the Meditations,Descartes famously considers an even more radical possibility: theentire material world could be an illusion. You could be a disembodiedghost dreaming that you have a body or a disembodied mind beingdeceived by a malicious and powerful demon into believing that there isa material world.

Philosophers usually discuss outlandish thoughtexperiments such as these in order to raise questions about thepossibility of knowledge. Does knowledge require absolute certainty?That is, if I cannot definitively rule out the possibility that I'm abrain in a vat, can I ever know that I have a body? Is there anythingthat could count as evidence or a reason to believe that I'm not abrain in a vat?

The Warner Bros. website for The Matrixactually contains a number of excellent philosophical essays about thefilm. (Some are written by very prominent philosophers.) My favoritesare probably the ones written by David Chalmers and James Pryor, but they're all terrific.

If the Sun were to explode we could not know of it until eight minutes later,

If the Sun were to explode we could not know of it until eight minutes later, because that is how long it takes for light to travel from the Sun to Earth. So for eight minutes we would see an unexploded Sun, while the real sun would be exploded. It follows that we do not see the real Sun. But everything we see must be later than reality, because of the time it takes light to travel from reality to our eyes, so nothing we see is real. Can this be true?

We do not see the sun as it is at the moment of our seeing; but it certainly does not follow from this that we do not see the real sun.

There are all kinds of delays, transformations and distortions of sensual evidence. This has led some philosophers (notably Descartes) to doubt whether sensual evidence can be relied upon to give an accurate 'picture' of how the world is. Descartes even went so far as to doubt whether things outside of him really existed at all. But the fact that I can doubt something is different from evidence for it not being real. Accordingly, Descartes did not argue that the general unreliability of sensual evidence had, as an implication, that what was sensed was not real.

So, in your example, that the sun has ceased to be in the meantime (and therefore is no longer a real thing) is a problem about the reliability of our sensible knowledge in the present, but not a problem about the reality of what is sensed.

Is there a way to perceive the real world? Thanks.

Is there a way to perceive the real world? Thanks.

The obvious answer to your question is Yes. You perceive the real world by opening your eyes, listening, touching, etc. The real world is composed of trees and traffic lights and eagles, and you perceive them using your organs of sense.

If you think this is a cop-out, that somehow the world I described isn't "the real world", then you'll have to say more about what "the real world" is and about why you think the world I described isn't really "real".

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