Figure-Ground Relationship
According to this principle, we tend to segment our visual world into figure and ground.
Figure is the object or person that is the focus of the visual field, while the ground is the background.
As the figure below shows, our perception can vary tremendously, depending on what is perceived as figure and what is perceived as ground. Presumably, our ability to interpret sensory information depends on what we label as figure and what we label as ground in any particular case.
The concept of figure-ground relationship explains why this image can be perceived either as a vase or as a pair of faces.