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From: mmt@dciem.UUCP (Martin Taylor)
Newsgroups: net.philosophy
Subject: Re: Why do mirrors reverse left & right, not up & down?
Message-ID: <671@dciem.UUCP>
Date: Fri, 3-Feb-84 17:43:23 EST
Article-I.D.: dciem.671
Posted: Fri Feb  3 17:43:23 1984
Date-Received: Fri, 3-Feb-84 20:32:20 EST
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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
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