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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...

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 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...

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...

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...

What is a relational property? In an earlier question about a car driving down a road and appearing to get smaller with distance, Prof. Moore wrote that this appearance is a relational property of the car, as opposed to the real size of the car, which is an intrinsic property of the car --- and what I see is this relational property. [See, http://www.amherst.edu/askphilosophers/question/548.] But it is clear to me that what I see is a small car: how can a small car be a relational property, whatever that is?

A relational property is a property a thing has only in virtue of how it is related to something else. A common example is fatherhood . Whether I'm a father depends upon my relation to something else, namely, my child (if I have one). So the claim is that, in so far as the car appears to be small, its apparent size is merely a relational property. It is, for example, a matter of how large a portion of your visual field the car occupies. Moore's point was that it's not the car itself whose size changes as it recedes, but only how large a portion of your visual field the car occupies. Now, it's certainly true that one is given to describing this phenomenon by saying that the car gets smaller. But, of course, one doesn't really think the car itself gets smaller, and there is an obvious sense in which it seems to remain the same size but to get farther away. To suppose there is some contradiction here is to suppose that how large an object appears to be is a direct function of how much of the visual...

Are you as Philosophers allowed to say that the rock on my desk is red? For we really don't know. We perceive it as red but what if our eyes are not showing us what is really there? For all we know, everything could be black and white.

There are many serious questions along these lines. The redness of your rock seems to be a property the rock has as it is in itself, but early modern philosophers, beginning with Descartes but perhaps most famously Locke, questioned whether that is so. There are many sorts of alternative views, but perhaps the most common nowadays is the so-called dispositional theory of color: Colors are relational properties, on this view; for a thing to be red is for it to tend (under normal conditions) to cause certain kinds of sensations in perceivers. If that is right, then there is a way in which color is only in our minds. See, as usual, the Stanford Encyclopedia for more on this issue. Note, however, that this issue isn't really best formulated as one about whether we know the rock is red. The issue is one about what it is for the rock to be red. On either view, we can and often do know such things. Or, at least, whether we can or do is an independent question.

Does the human mind perceive sight in 3 dimensions, or do we actually see in 2 dimensions, where depth perception and distance really don't exist in our mind? For example, I am looking at a bridge 100 yards away, I place my finger directly in front of the bridge. Now in the external world there exists a finger an arm's length away from my eyes, and a bridge 100 yards away. If the picture that occurs in my mind is a 2 dimensional picture then my finger and the bridge are located on the same plane in my mind, and distance would not truly exist in my perception. But if the mind's perception of sight occurs in 3 dimensions, like a hologram, then the picture I receive through sight must occur in a three dimensional space in my mind, where distances must be in the same but smaller ratios as exist in the external world. Here occurs a problem. If our perception of vision occurs in smaller but equal ratios of three dimensions, then the same object would have two sizes: That which exists in my mind (extremely...

We need to keep some things straight here. The "picture" that occurs in your mind, if there is such a thing, is a representation . You don't have a single object, the bridge say, which has two sizes. The bridge isn't both in the external world and in your mind. It's just in the external world. If there is a "picture" of the bridge in your mind, then it is a picture of the bridge, not the bridge, and the fact that it has a different size from the bridge is hardly surprising. More generally, whether the representation is three-dimensional or not (and there is some empirical evidence that it is, actually) has nothing particular to do with whether it is capable of representing three dimensions. Think of a map. On the map, dots represent towns and the relative distances between little dots represent distances. If it's a topographic map, then there may be lines that represent height. The map represents three dimensions even though it is essentially two dimensional. It's important to be...

On what basis can we claim somebody is delusional? Assuming objective, True Reality(tm) exists, but is not directly knowable and is only knowable through mediation of our senses, how do we have any solid footing for deciding one person's senses are defective compared to another's? Two thought experiments to illustrate this idea: Assume I am alone in a room, and I see a purple monkey swinging from the lamp. I perceive this odd sight and may, or may not, decide that I'm hallucinating based on my previous experiences. If somebody else comes into the room, and I ask them what they see, if they agree with me, then odds are better that we are both seeing accurately, but if he disagrees with me, then he may be blind to the monkey, or I may be imagining it. Adding more people will get us a consensus view, but doesn't really prove anything in more than a statistical way. Who is delusional, and who is seeing truly? Or, assume I am the one-eyed man in the land of the blind. Like the monkey-seeing fellow in...

There seems to be a pattern of argument here that needs to be questioned. It is: (i) Method M for reaching judgements isn't completely reliable; therefore, (ii) method M can't be trusted. The conclusion simply doesn't follow. Method M might be very reliable, in which case it can be trusted to a high degree. I doubt "group opinion" is the only method available to us for distinguishing hallucination and illusion from perception. (For example, in the land of the blind, there might be ways of correlating your apparent perceptions with facts on which you can all agree, e.g., that there is a boulder in such-and-such a location that wasn't there yesterday.) But it might be quite a good way of drawing the distinction even if it isn't a perfect way.