The most fundamental Gestalt principle of organization is that we tend to distinguish
between figure and ground. The figure is the thing that you see while the
ground has no specific shape. The figure also tends to appear closer, while the ground
appears less localized than the figure. Finally the shape that has more meaning is also
usually seen as the figure, whereas the shape with less meaning is usually seen as the
ground.
Take a look at the black and white figure at the top of the page. Because there are two
meaningful organizations, a vase, or two faces, it is easy to shift between the two
perceptual organizations. Either the vase or the faces can be perceived as the figure.
Now look at the stereogram. This stereogram was made from the black and white image of the
vase/faces. But the major difference is that the faces now appear closer than the vase.
This has the effect of forcing the faces to be the figure and preventing you from seeing
the vase.