Space is the complement of shape. Change in use of space affects our perception of shapes (and the reverse is also true). In the image below, the shapes on the left side are placed to create negative space. The same shapes are arranged on the right side to create a positive space and form an implied shape in the center.
The relationship between shape and space affects our perception of the image. For example, in the image below, is the shape on the left side the same size as the shape on the right side of the image? They are drawn to the same size, but, the differences in the space that the shape resides in create the visual illusion that the image on the right side is slightly larger.