I must confess this makes my head spin, which makes me suspect there's something wrong with the question. Richard Dawkins used it to pose this to university applicants, to see which "would probably be fun to teach…it's a provocative puzzle…but is it in psychology, in physics, in philosophy, in geometry, or just commonsense?"

It certainly looks as though reflections are flipped left-right. Writing reads backwards, but not upside down. Your left cheek becomes your right cheek, but you're not standing on your head.

One approach might be a technique called "ray tracing", where the paths of light are mapped through space. Movie makers use it to create realistic scenes, generating images that appear natural in the surrounding light. If we apply it to our mirror problem, we would track light hitting your face, bouncing off the mirror, and into your eyes.

It's a technical solution, but not satisfying. Something is leading us astray, and we need to look at psychology to understand it better. Our brains use simple rules to explain the world. It's quick, but sometimes misleading. In real life, we commonly see objects moved up, down, left, right, etc. We see them rotated, but reflection is relatively rare, especially before we had good mirrors.

A clue is that reflected writing is reversed, regardless of orientation. If you hold the page sideways, it's still reversed. You understand the terms "top" and "bottom", but they're to a mirror they're irrelevant.

We interpret images as reversed, left to right, but really they're reversed from front to back.

If you face a mirror and point to one side, you and your reflection are pointing in the same direction. Point to the front, and you are pointing in the opposite direction. Point upwards, and you both point in the same direction. In all cases the direction reverses only when you point towards or away from the mirror.

Mirror writing is hard to read, but some people write that way deliberately. A notable example was Leonardo da Vinci, who only using standard writing when he intended it to be read by others. Why, is unknown, but it might have been because he was left handed, and didn't want to smudge the ink.

Response by: Rod taylor, Fuzzy Logic

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