If you put a banner in front of a mirror, it flips the left-right axis, but not the down-up axis. Why don't all directions have equal merit in the universe? How does this relate to the time dimension -- is it also special in the same sense?

The reason I'm asking this is that my intuition is that every dimension should be the same. That left and right should be the same as up and down, and that forward in time should be the same as forward to the right. But nature seems to treat different dimensions differently. Here I gave the example of mirrors, that treat left and right differently from up and down. One could easily have given time: you can go forward in time, but backwards is impossible. Why does nature treat different dimensions differently? And in particular, who would a mirror, which is perfect symmetric, treat different dimensions differently? Why does nature "pick favorites" when it comes to dimensions, if it has no a priori reason to give different roles to each dimension?

  • 2
    Can you please develop this a little bit? Maybe unpack some of the motivations and context which led you to formulate this question? Perhaps you could also tell us a little bit about your acceptance test here: what specific thing are you looking for someone here to explain to you? (We also encourage in certain cases, as an act of good faith, demonstrating topicality -- that is, identifying how the community here explaining this to you helps you become a better student of philosophy.)
    – Joseph Weissman
    Commented Oct 21, 2011 at 20:53
  • Very well, I will edit some.
    – Rom
    Commented Oct 21, 2011 at 20:54
  • 1
    @Rom : related question would be why mirors swap left and right, but not top and bottom? But you have to ask it in physics.se.
    – jimjim
    Commented Oct 22, 2011 at 5:20
  • This doesn't have to do with physics. Physics would explain how the optics of it work; but I'm looking for the "why". Why should left and right be different from up and down? Why should forward in time be different than forward to the left.
    – Rom
    Commented Oct 22, 2011 at 14:04
  • 1
    Do a web search: short answer-the mirror is -not- switching left and right. The right side of the mirror shows the right side of your face, right?
    – Mitch
    Commented Oct 22, 2011 at 16:57

2 Answers 2


It doesn't flip the left-right axis or the up-down axis. It flips the front-back axis, which is special because your front is closer to the mirror than your back.

You think it flips the left-right axis because you imagine yourself superimposed on your mirror image by walking into the mirror and then turning around. But it is this imagined turning around that reverses your left-right axis.

If you imagine yourself simply walking into the mirror, the reversal will be front-back only. If you imagine yourself superimposed on your mirror image by diving into the mirror such that your head lands where the mirror image's feet are, your left hand will superimpose on the mirror iamge's left hand, and vice-versa for right. Except now the image will be reverse on the up-down axis.

So, it's because human beings have left-right symmetry and are easily imagined turning around but not so easily imagined diving onto their heads that a mirror appears to reverse a human being left-right. It is actually front-back.

  • 1
    Another way to make this manifest is to not consider "left" and "right", but the cardinal compass directions: "north", "south", "east", and "west". If any of these are pointing into/out of the mirror, that pair is the pair which gets reversed in the mirror. Commented Oct 21, 2011 at 22:31

The question is based upon an incorrect premise; mirrors do not flip left and right.

Furthermore, this is a matter of physics (and optics), not philosophy.

The philosophical portion of the question-- the intuition that all dimensions should be "treated equally", and that nature fails to do so-- follows from this incorrect assumption.

  • Incorrect is subjective. For example my left and right can be the opposite of yours.
    – Rom
    Commented Oct 22, 2011 at 14:12
  • 6
    No, it really is incorrect. A mirror does not reverse left and right. There is a great deal of scientific literature on this misconception; Martin Gardner, Isaac Asimov, and Richard Feynmann have all written on it, to name just a few. In fact, even the Wikipedia article on "mirror image" (en.wikipedia.org/wiki/Mirror_image) discusses this misconception. Commented Oct 22, 2011 at 14:21
  • There is a great deal of literature on the "misconception" of religion. It doesn't mean that it's wrong, it simply means that there are people who can't admit their subjective limitations.
    – Rom
    Commented Oct 22, 2011 at 16:26
  • 1
    @Rom: exactly. And a mirror -doesn't- act like people's faces.
    – Mitch
    Commented Oct 22, 2011 at 16:59
  • 7
    @Rom: it's the equivalent of an optical illusion. It doesn't make sense to treat is as a fact, and then ask a question predicated upon it being correct. If you had asked "Why does a pound of lead weigh more than a pound of gold? Nature shouldn't discriminate between elements" the only possible response would be to say that a pound of lead doesn't weigh more than a pound of gold, and that the question was based upon a misconception. Mirrors treat left and right the same exact way they treat up and down. Really. Commented Oct 22, 2011 at 17:43

Not the answer you're looking for? Browse other questions tagged .