Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83; site dciem.UUCP Path: utzoo!dciem!mmt 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 References: <543@bbncca.ARPA> Organization: D.C.I.E.M., Toronto, Canada Lines: 31 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!mmt