Entering the Third Dimension.
The third dimension, the spatial depth of an image or a moving scene, is lost when it is reproduced on a two dimensional medium such as paper, a cinema screen or a computer monitor.
Hence the representation appears flat, even though our brains normally succeed in making guesses about how far an object in the image might be away from the viewer.
Seeing a real scene (as opposed to a 2D image of this scene) with both eyes provides extra information for the brain, as each eye has a slightly different point of view on the world.
So in everyday life the resulting small differences in these two images allow us to make guesses how far an object in space is really away from us. This principle which really makes a world of a difference for most of us, has been known as "stereoscopy" since the 19th century.