If you read the image as two people facing one another, then the faces become the figures, or positive shapes, while the white area between the two becomes the ground, or negative space. This is one aspect of an artist's decision-making process that is often overlooked. Artists are always conscious of the relationship between the figure and the ground, or the positive and negative shapes in their work. This idea is an important consideration when examining works of art as well. Artists also make use of our emotional and psychological responses to shapes.
Squares are hard and stable and even.
Look at Josef Albers' painting on page 17. Circles are constant. Look at M. C. Escher's piece on page 41. Your eyes follow the circle around and around without ever meeting a sharp point or corner where you must then decide which way to go. A triangle pointing upward is solid and centered.