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. Beyond geometric shapes, there are all of the shapes based on nature to consider. These are called organic shapes.