Thursday, December 09, 2010

"Objects in Mirror Are Closer Than They Appear"

... so it says, at least, on my passenger side mirror.

(image from

I've been worrying though, are they closer than they appear?  This might seem a strange thing to worry about, but I refuse to be thus consoled.

Here's a case for saying that objects in the mirror are closer than they appear: The mirror is slightly convex so as to give the driver a wider field of view.  As a result, the expanse of mirror reflecting light from the object into my eye is smaller than the expanse would be if the mirror were flat.  Thus, the size of the object "in the mirror" is smaller than it would be in a flat mirror.  If we assume that flat mirrors accurately convey size, it seems to follow that the size of the object in the mirror is inaccurately small.  Finally, apparent distance in a mirror is determined by apparent size in a mirror, smaller being farther away.

The argument for the other side is, at first blush, much simpler: Objects in the mirror are no closer than they appear, at least for me, because as an experienced driver I never misjudge, or am even tempted to misjudge, their distance.

Now both of these arguments are gappy and problematic.  For example, on the first argument: Why should flat mirrors be normative of apparent size?  And why shouldn't we say that the object is larger than it appears (but appearing the right distance away), rather than closer than it appears (but perhaps appearing the right size)? That is, why does it look like a distant, full-sized car rather than a nearby, smallish car?

You might be tempted to mount a simpler argument for the "closer than they appear" claim: A naive mirror-user will misjudge the distance of objects seen in a slightly convex mirror.  The naive mirror-user's misjudgments are diagnostic of apparent size -- perhaps they are based primarily on "appearances"? -- and this apparent size does not change with experience.  The experienced mirror-user, in contrast, makes no mistakes because she learns to compensate for apparent size.  But this argument is based on the dubious claim that the experience of a novice perceiver is qualitatively the same as the experience of an expert perceiver -- a claim almost universally rejected by contemporary philosophers and psychologists.  It's also unclear whether the naive mirror-viewer would make the mistake if warned that the mirror is convex.  (Can apparent size in a mirror be contingent upon verbally acquired knowledge of whether the mirror is slightly convex or concave?)

Should we, then, repudiate the manufacturers' claim, at least as it applies to experienced drivers?  Should we, perhaps, recommend that General Motors hire some better phenomenologists?  Well, maybe.  But consider carnival mirrors: My image in a carnival mirror looks stretched out, or compressed, or whatever, even if I am not for a moment deceived.  Likewise, the lines in the Poggendorff Illusion look misaligned, even if I have seen the illusion a thousand times and know exactly what is going on.  Things look rippled through a warped window, no matter how often I look through that window.  Perhaps you, too, want to say such things about your experience.  If so, how is the passenger-side mirror case different?

Here is one way it might be different: It takes a certain amount of intellectual stepping back to not be taken in by the carnival mirror or the Poggendorff Illusion or the warped window.  The visual system, considered as a subset of my whole cognitive system, is still fooled.  And maybe this isn't so for the passenger-side mirror case.  But why not?  And does it really take such intellectual stepping back not to be fooled in the other cases?  Perhaps there's a glass of water on my table and the table looks warped through it.  I'm not paying any particular attention to it.  Is my visual system taken in?  Am I stepping back from that experience somehow?  It's not like I just ignore visual input from that area: If the table were to turn bright green in that spot or wiggle strangely, I would presumably notice.  Is my father's visual system fooled by the discontinuity between the two parts of his bifocals?  Is mine fooled by the discontinuities at the edge of my rather strong monofocals as they perch at the end of my nose?  And what if, as Dan Dennett and Mel Goodale others have suggested, there are multiple and possibly conflicting outputs from the visual system, some fooled and some not?

Can we say both that objects are farther than they appear in passenger-side mirror (in one sense) and that they aren't (in some other sense)?  I'm inclined to think that such a "dual aspect" view in this case only doubles our problems, for it's not at all clear what these two senses would be: They can't be the same two senses, it seems, in which a tilted penny is sometimes said to look in one way round and in another way elliptical -- for what would we then say about the tilted penny viewed in a convex mirror?  We would seem to need three answers.

Hey, wait, don't drive off now -- we've only started!


Kurt said...

I'm guessing it's just legal ass-covering, the mirror disclaimer. Someone would otherwise use the peculiar optics of rear view mirrors as grounds for litigation.

Eric Schwitzgebel said...

Yes, that seems likely!

G. Randolph Mayes said...

Great post. I found myself thinking about the classic video Living in a Reversed World ( and wondering how, after years of driving with them, we would experience rear view mirrors that produced a variety of other distortions instead. My guess is that you would be able to make the same set of phenomenological observations about them. I guess the practical question is whether the standard warning is an adequately clear abbreviation of "Warning: Objects in mirror are closer than they appear to drivers whose visual systems do not automatically compensate for the distortions produced by convex mirrors." Next up for epistemological analysis: "This product contains chemicals known to the State of California to cause cancer and birth defects or other reproductive harm."

Eric Schwitzgebel said...

Thanks for the comment and the link, Randy. Yes, I think all these issues are connected!