rrizzo@bbncca.ARPA (Ron Rizzo) (01/31/84)
How about "doing" some philosophy on the net? Can anyone provide a convincing account of why mirrors perversely do such things? Cheers, Ron Rizzo
rjnoe@ihlts.UUCP (01/31/84)
You must be kidding. Mirrors do not reflect left/right. Mirrors simply reflect light at angles (nearly) equal to the incident angle of that light. You perceive it that way because of your gravitational bias. Stand on the wall and you'll be saying instead, "Why do mirrors reflect up/down and not left/right", assuming you do not rename directions because of your new perspective. I suppose one would be less likely to think this way if one had only one eye, but it's hard for me to say. Roger Noe ihnp4!ihlts!rjnoe
mmt@dciem.UUCP (Martin Taylor) (02/01/84)
How about "doing" some philosophy on the net? Can anyone provide
a convincing account of why mirrors perversely do such things?
Cheers,
Ron Rizzo
==========
It's hard to see why this relates to "philosophy". Mirrors don't
reverse left-to-right or top-to-bottom. They reverse front-to-back.
The only "philosophical" question is why people think they reverse
left to right? Presumably it's because the viewer sees "another"
person out there who moves a hand identified as the left hand when
the viewer moves the right hand. Is that philosophical enough?
--
Martin Taylor
{allegra,linus,ihnp4,uw-beaver,floyd,ubc-vision}!utzoo!dciem!mmtrrizzo@bbncca.ARPA (Ron Rizzo) (02/01/84)
Martin Taylor's reply begs the question. Let's say the mirror reversal is front-to-back. The question once again is why does the resultant front-to-back reversed image in the mirror exchange left and right and not top and bottom? (It's not necessary for the viewer to perceiver a "person" in the image: the left/right exchange is still perceptible even when viewing the mirror image as an abstract shape. One could also put a guitar in front of the mirror and observe the same phenome- non.) The question isn't trivial nor are its "answer(s)" easy or evident. It was the exclusive subject of a "philosophically-oriented" course given at MIT a few years ago. As to whether the question is "philosophical", if the attempt to answer it causes us to think about assumptions or basic ideas we rely on, then the question certainly has philosophical import. But thanks for the reply; I hope others will be forthcoming (this mailing list doesn't show much activity recently).
rrizzo@bbncca.ARPA (Ron Rizzo) (02/02/84)
Let's remove images of persons or things that have fronts & backs from the debate by substituing the following paraphrase: Alice climbs up on the mantle of the fireplace, and lo! the drawing room appears backwards [oops--that word! Of course, she means right/left switched] in the looking glass. How curious, she thinks.
rrizzo@bbncca.ARPA (Ron Rizzo) (02/02/84)
THERE'S MORE TO MIRRORS THAN MEETS THE EYE To further investigation of mirrors, I'll create fictitious persons to make up for the lack of real ones posting to this mailing list & submit the following imaginary dialogue: PSEUDO-TAYLOR: What a crock. OK, eliminate images of persons, or even distinct objects. Our vision contains an implicit model: the visual field is a planar projection, or better, a projection onto a surface whose curvature derives from the eye's structure. As seen, the drawing-room is a projection surface. Now just apply the front- to-back mapping to that. That's all. RIZZO: I appreciate your point. I do that agree your "front-to-back mapping" accounts for why points on the reflected object correspond to points in the image that appear to be on the "wrong side". For- mally restated, your "front-to-back mapping" is an automorphic (maps an object onto itself) transformation involving a 180 degree rotation about a top-to-bottom axis. In effect, yours is a proposal to trans- fer the explanatory burden from one distinction, left/right, to an- other, front/back. But a problem arises. Why is the axis of rotation a top-to- bottom one? Rotation through a left-to-right axis (or any other axis coplanar with the top-to-bottom axis) is equally a "front-to-back mapping". In fact, the terms "front" & "back" don't contain the extraneous notions "top" & "bottom" : which axis you use ought to be irrelevant. So the offered "mapping" can't be called "front-to-back" since those terms can't carry all the explanatory burden. PSEUDO-TAYLOR: Alright, so call it "mapping X". You still know exact- ly how it works. RIZZO: But what is "X" ? I'd like to know, because this "X" manages to get mirrors to do something very strange: to posit a particular axis that determines their behavior when it's hard to see from the properties of glass, light, & 3-dimensional space why mirrors would favor such an axis. Why do mirrors use a vertical axis, & not, say, a horizontal one? (A brief pause in the conversation ensues. Then:) PSEUDO-TAYLOR: They don't. We do. RIZZO: Intriguing! Why do we? PSEUDO-TAYLOR: Because our vision is binocular. Binocularity creates handedness. RIZZO: Hmm. Yet, close an eye. You're monocular. Your open eye still comprehends "left" & "right". PSEUDO-TAYLOR: No! your single open eye is binocular. Binocularity is not a matter of how many eyes you're using. It refers to a model of HOW to see (how to visually interpret the world), which we LEARN as we experience the world in our first years. Having two eyes may be a pre- condition for acquiring binocularity, but once it's achieved, even a single eye sees "as two". RIZZO: Now that's pretty impressive. (Another pause occurs. Then continuing:) RIZZO: Suppose our Alice has experienced a "change". Her eyes now lie on her face on a vertical, not a horizontal, axis (poor thing!). With her back to the fireplace, she surveys the drawing-room. The green chair is in the corner to her "bottom" (on her bottom eye's side of the visual field, i.e. our left) and the mahogany writing-desk in the corner to her "top". Once again she clambers up onto the mantlepiece and gazes into the looking-glass. "Oh, dear!" she cries. For in the glass the mahogany writing-desk appears at the "bottom" (our left) and the green chair at the "top". It's seems like we're dealing with an invariant (right/left)? Perhaps the above scene misconstrues binocularity. If so, tell me more about it, so I can correctly understand it. Any real persons care to join this debate?
ags@pucc-i (Seaman) (02/02/84)
Mirrors do NOT reverse left & right, OR up & down. Only back & front (i.e. perpendicular to the mirror). If you WANT to reverse left & right, that can be arranged. Set up two mirrors at an angle, like this: /\ / \ / \ / \ / \ ^ ^ ^ | | | | | | line of sight If you look into these mirrors (placed approximately at right angles) and you are facing north, then you can wink your EAST eye and see your image wink its WEST eye. -- Dave Seaman ..!pur-ee!pucc-i:ags "Against people who give vent to their loquacity by extraneous bombastic circumlocution."
brian@digi-g.UUCP (Brian Westley) (02/02/84)
"Why do mirrors reverse left & right, not up & down" Mirrors reverse, period. You are trying to map the most natural frame of reference (i.e. imagining yourself stepping through the mirror into the 'other' person's shoes) onto your reflection, when the two other mappings (pitching forward into a handstand into the 'other' person's shoes, or simply stepping forward into his shoes without turning around to face to same way) now seem to result in a mirror reversing up & down, or back & front, respectively. You'll notice that in these mappings, left & right are not reversed. If humans were not bilaterally symmetrical, this would be more obvious. Fiddler crabs aren't confused by mirrors. (signed) Merlyn Leroy
ark@rabbit.UUCP (Andrew Koenig) (02/03/84)
This is a trick question. Defining terms carefully makes the problem go away. Imagine yourself standing in front of a full-length mirror. The reflection of your right hand is nearest the right edge of the mirror, and the reflection of your left hand is nearest the left edge. The reflection of your head is nearest the top edge, and the reflection of your feet is nearest the bottom. In what way, then, can a mirror be said to reverse left and right, any more than it reverses top and bottom? Well, now, you may say, suppose there's a person standing behind you. Her right hand is also nearest the right edge of the mirror and her left hand is nearest the left edge. If you turn around to look at her, though, her right hand will be on your left and her left hand will be on your right. In that sense, what you see when you look directly at her is reversed left for right compared to what you see in the mirror. However, that has nothing to do with the mirror. Rather, it has to do with what you did when you turned around to look directly at her. You turned around a vertical axis. If instead you had turned around a horizontal axis (by standing on your head), her left hand would still be nearest to your left hand and her right hand would still be nearest your right hand. Her head, though, would now be near your feet and her feet near your head.
laura@utzoo.UUCP (Laura Creighton) (02/03/84)
I am blind in one eye. I always (or at least since before I entered school) have been this way. I have never been able to really feel the difference between right and left. It would be damn useful, since everybody else uses it, but the best I can do is know that my left hand is the one with the big scar on it, and the right one is not. I can look at my hands very quickly and tell left from right, so I don't run into all that much trouble in the world. Growing up this was a real pain (I hadn't had the accident with the knife yet). I have no real sense of how letters follow one from another. Thus I can read english in right-to-left every bit as quickly as from right-to-left. All I can see is a convention which everybody follows. Did you know that it never occurred to me that mirrors reverse at all until you mentioned it? If mirrors reverse, then I bet that this is because all of you guys have binocular vision. It makes perfect sense to me. But, before you all go out and decide to blind yourself in one eye, I will suggest that it makes a difference that I was born this way an grew up without left/right distinctions and also that in a lot of ways being blind in one eye is a pain in the you know where. Going to school not being able to tell left from right is not fun. Depth perception is funny. I can tell up from down real well. But gravity pulls one down. In Australia it doesn't pull you up. Down is where I would go if I fell right now, and up is the opposite direction. This doesn't require binocular vision. Problem solved? Or do you really want to discuss Epiphenomenalism? -- Laura Creighton (NOTE NEW ADDRESS) utzoo!laura
ags@pucc-i (Seaman) (02/03/84)
If you face north and look into a mirror, you see your image facing south. If you raise your east hand, your image also raises its east hand. This shows that the mirror reverses only back-front, not up-down or left-right. The confusion comes when you try to mentally superimpose your real self on your mirror image. You do this by a rotation instead of a reflection. This has the effect of reversing BOTH left-right and up-down AT THE SAME TIME. The disparity between the rotation and the reflection is what causes people to think that left and right are reversed, when in fact they are EXPECTING to see left-right reversed and find that it ISN'T. Try the two-mirror experiment which I suggested before to see the difference. This allows you to simulate a true rotation with mirrors, rather than a simple reflection. -- Dave Seaman ..!pur-ee!pucc-i:ags "Against people who give vent to their loquacity by extraneous bombastic circumlocution."
billp@azure.UUCP (Bill Pfeifer) (02/03/84)
* >> How about "doing" some philosophy on the net? Can anyone provide >> a convincing account of why mirrors perversely do such things? Mirrors aren't perverse, and they do no such thing. They simply reflect light back the same way they receive it. Think of the image in the mirror as that on a photographic slide. Orient the slide such that if you look through it, it matches the original scene. This corresponds to you standing behind the mirror. In order to see the mirror's reflected image, however, you must move to the other side of the mirror and rotate your body 180 degrees. This corresponds to flipping the slide over. The way you rotate you body or flip the slide, determines whether you, not the mirror, exchange left & right or up & down. If you simply turn around, this corresponds to rotating the slide about its vertical axis. You have now exchanged left & right, but not up & down. If you were to stand on your head and look at the image in the mirror, this would correspond to rotating the slide about its horizontal axis. You have now exchanged up & down, but NOT left & right. Bill Pfeifer {cbosg,decvax,harpo,ihnss,ogcvax,pur-ee,ucbvax,zehntel} !tektronix!tekmdp!billp
rlr@pyuxn.UUCP (Rich Rosen) (02/03/84)
Put a mirror on the ceiling. The room will not be reversed left/right at all, just upside down. No, I don't spend my time in rooms with mirrored ceilings... Not often, anyway. -- Pardon me for breathing... Rich Rosen pyuxn!rlr
mmt@dciem.UUCP (Martin Taylor) (02/04/84)
REAL-TAYLOR here!
Rizzo's pseudo-Taylor follows my argument up to the point where there
is a pause in the conversation, but then runs off down an Alician
garden path.
Pseudo-Taylor is correct in saying that mirrors don't reverse left to
right, we do. But that has nothing to do with binocularity. It has to
do with the fact that up-down is a distinction we need to make in everyday
life (everyday over billions of years) whereas left-right is needed for
almost nothing except reading. When we place ourselves (or a guitar) in
the perceptual place of the mirror, we normally image ourselves as turning,
but staying on our feet.
The inversion is an inversion of the handedness of the coordinate system.
It is entirely up to you which coordinate you choose to think of as the
reversed one. Physically, the mirror maps Z into -Z (where Z is perpendicular
to the mirror), and does nothing to X or Y (parallel to the mirror plane).
If we had evolved in free-fall, we probably wouldn't even think about
left and right in this connection.
Think of reading mirror writing. It's seen as left-to-right, but it's
exactly the same as what you see looking out through a shop window
(writing seen from behind). The choice of which inversion to see is
uniformly yours (I don't mean a conscious choice. There may be some
psychological experiments worth doing to see whether people can be
influenced in which dimension they see reversed, but I don't see
any philosophy).
--
Martin Taylor
{allegra,linus,ihnp4,uw-beaver,floyd,ubc-vision}!utzoo!dciem!mmtcolonel@sunybcs.UUCP (George Sicherman) (02/05/84)
You could write to J. L. Borges, an expert on the philosophical implications of mirrors. He might be able to clear up the problem. It sounds something like the Headless Self problem. If a real person stands facing YOU, then his right hand faces your left. But this is merely by custom. If most of your acquaintances go walking on their hands, then their right hands will face your right hand, and your reflection will seem top-to-bottom inverted by comparison. In other words, it's not just the reflection's fault. If your reflection were to look out at you and at somebody standing next to you, he'd think BOTH of you were reversed from left to right. Hope this helps. :-) Col. G. L. Sicherman ...seismo!rochester!rocksvax!sunybcs!colonel
ags@pucc-i (Seaman) (02/06/84)
> RIZZO: I appreciate your point. I do that agree your "front-to-back > mapping" accounts for why points on the reflected object correspond > to points in the image that appear to be on the "wrong side". For- > mally restated, your "front-to-back mapping" is an automorphic (maps > an object onto itself) transformation involving a 180 degree rotation ^^^^^^^^ > about a top-to-bottom axis. In effect, yours is a proposal to trans- > fer the explanatory burden from one distinction, left/right, to an- > other, front/back. Wrong. The front-to-back mapping is not a rotation at all, but a reflection. There is no top-to-bottom axis, or left-to-right axis. There is only the plane of the mirror, which represents the plane of reflection. It is exactly this tendency to replace a reflection with a rotation which confuses people into thinking there is a left-right reversal. > RIZZO: ... Why do mirrors use a vertical axis, & not, say, a horizontal one? > > (A brief pause in the conversation ensues. Then:) > > PSEUDO-TAYLOR: They don't. We do. > > RIZZO: Intriguing! Why do we? > > PSEUDO-TAYLOR: Because our vision is binocular. Binocularity creates > handedness. PSEUDO-TAYLOR was on the right track, but he came up with the wrong explanation. The real reason that WE (not mirrors) use a vertical axis rather than a horizontal one is that: (1) It is more natural. We turn around much more often than we do cartwheels. (2) Humans (and many objects) are approximately symmetric left-to-right, but not even close to symmetric top-to-bottom. If you lie down horizontally and look into a mirror, it will seem more natural to use a rotation about a horizontal axis, since this produces an image more nearly in agreement with what you see in the mirror. -- Dave Seaman ..!pur-ee!pucc-i:ags "Against people who give vent to their loquacity by extraneous bombastic circumlocution."
edhall@randvax.UUCP (02/07/84)
-------------------------------------- Well, Laura, generalization can be dangerous. I have very good binocular vision, and still have one hell of a time telling left from right. I always have to *think* about it, and it helps a lot if I look down and see which arm has my wristwatch on it (my left). And I figure out which arm to put my watch on by the untanned band from its shadow. On the other hand, a good friend of mine, who is blind in one eye, has much less of a problem telling her left from her right. I'd think that the difference between right and left would be fundamental, yet my perceptual system (along with Laura's and a whole lot of other people's) just doesn't seem to be set up to distinguish it. This leads to some other questions about perceptions and dif- ferences in people's innate ability to perceive certains things. It is quite possible that reality is just plain *different* for different people, and that some disagreements between individuals are simply insoluble. If I can't see your point of view, perhaps my nervous system is simply incapable of seeing things that way. Or perhaps right/left is simply a special case. It's food for thought, though. -Ed Hall decvax!randvax!edhall
rrizzo@bbncca.ARPA (Ron Rizzo) (02/07/84)
Thanks for all the replies! This mailing list still lives (but a few more false starts like my "Mirrors...." may KO it?). As I crawl back from the thin branch I've been perching on lately, I resolve to eschew verbosity & shun vainglorious display....
rlr@pyuxn.UUCP (Rich Rosen) (02/07/84)
I haven't seen such an anthropocentric discussion (outside of net.religion) in my life! Mirrors reversing left to right because of OUR binocular vision? If it wasn't a person in the mirror it wouldn't be reversed? O.K. Picture yourself and your Rubik's cube (assume each side of the cube has been made uniform in color) walking in a three dimensional grid. (Assume x axis is the horizontal left-right line in front of you, y axis is the vertical line in front of you, and the z axis is passing through you from front to back; actually, assume that your cube is standing on the y = 0 plane at a location x = 0, y = 0 [actually ranging from 0 to the height of the cube], and z = 3). Now suddenly a "mirror image" of your cube appears at x =0, y = 0, z = -3 (on the other side of the z = 0 plane that we will consider to be the "mirror"). You walk over to the other side of the "mirror" to meet your cube's doppelganger, and you discover (aha!) that the top and bottom sides of the cube (red and blue, respectively---I don't have a cube handy so please forgive any cubic inaccuracies) are on the top and bottom (as expected---after all, mirrors don't reverse up and down, do they?), the front and back sides (assume front to be that which faces the "mirror") are yellow and green respectively, on both sides of the "mirror", also as expected. But, look, if you assume the side facing the mirror on both sides to be the front, then there's a reversal of the left and right sides (again as expected)---in the original, the left side is white and the right side purple, but if you make the above assumptions, in the doppelganger the left side is purple and the right side is white. Think about it. When you walked to the other side of the mirror, how did you do it? You walked on the y = 0 plane. Now, let's pretend that you are Fred Astaire in Roman Holiday and that you can walk to the other side of the mirror ON THE x = 0 PLANE. But as you begin that walk, your perspectives of your original cube are altered. If you choose to still call the side facing the mirror the front, then when you "stand" on the x = 0 plane (let's use the position where your head is on the x > 0 side of the plane), the right side (purple) is now the top, the left side (purple) is now the bottom, the top (red) is now the left side, and the bottom (blue) is now the right. When you reach the doppelganger cube after walking along the x = 0 plane, again the left and right are reversed, while the top and bottom are not. The red side is the left side of this cube and the blue side is the right! But wait!! That's the same doppelganger cube as before---nothing has changed. Before, when you walked along the y = 0 plane, the red and blue sides were not "reversed" (in your perspective), but now they are!! It just depends on how you choose to "walk around" (in your mind) to the other side of the mirror. As someone already said, mirrors don't reverse left/right or up/down, they reverse back/front. *BUT*, if the human mind chooses to assume (note how many times I said "assume" in the proposition) that front is "that side which is facing the mirror", then the reversal will be perceived in one of the other two dimensions---height (up/down) or width (left/right). Because of our "horizontalist chauvinist perspective", we choose to walk around the mirror on the floor when we *re-visualize* the image in the mirror. If we played Fred Astaire, we could (if we chose to) visualize a different reversal---that which we call up/down when we are standing 'normally' would indeed be reversed. The philosophical question here really boils down to "Why do we have this horizontal chauvinist perspective?" What is going on physically is a one dimension reversal, which the brain translates from a back/front reversal to a left/right reversal. Physical questions abound: What would a mirror be like that reverses two dimensions? (If a mirror is perceived to reverse one dimension but not the other two, could it not be perceived geometrically that the other two dimensions are the ones that are reversed and not the first one?) One that reverses three dimensions? More? Are there mirrors in Flatland? (I haven't read it yet.) -- Pardon me for breathing... Rich Rosen pyuxn!rlr