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...

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.

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...

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...

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...

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...

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.

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...

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.