Imagine a child′s toy arrow, sticking by its rubber sucker to a mirror′s reflective surface. We can call the direction in which such an arrow would point the finwards direction (forwards into the mirror); and we can call the opposite direction boutwards (backwards out). When we look at things in a mirror, their images are apparently just as far finwards of the mirror as the things themselves are boutwards of it. For example, if we look at the tail of our arrow and cast our glance finwards, we see first the tail, then the head, then the mirror, then the reflection of the head, and finally the reflection of the tail. We can therefore say that a mirror reverses things in the finwards/boutwards dimension. Moreover, the straight line connecting each thing to its image passes perpendicularly through the plane of the mirror. Hence there is no plane, apart from that of the mirror itself, such that the apparent location of each thing′s image is just as far to the one side of that plane as the original is to the other. This means that the reversal in the finwards/ boutwards dimension is the only reversal of its kind to take place. In particular, there is no such reversal in any dimension at right angles to finwards/boutwards.