In motionless pictures this is less disturbing; in moving pictures every new movement to or from the background must remind us of the apparent distortion.

Moreover, the size and the frame and the whole setting strongly remind us of the unreality of the perceived space.

But the chief point remains that we see the whole picture with both eyes and not with only one, and that we are constantly reminded of the flatness of the picture because the two eyes receive identical impressions.

And we may add an argument nearly related to it, namely, that the screen as such is an object of our perception and demands an adaptation of the eye and an independent localization.

We are drawn into this conflict of perception even when we look into a mirror.