To start our discussion, we need to introduce three new terms: anatomy, proportion, and mass.

Anatomy: the skeletal and muscular structures of the body. List the bones and muscles that the artist tried to represent and ask whether the resulting figure is biologically correct. Did the artist actually observe a body or did he abstract or stylize the natural form which he observed?

Proportion: the size relationship of body part to body part. Are the eyes the correct size in relation to the length of the nose? What about the length of the arms and legs? Is the waist in the correct location?

Mass: the three-dimensional realities of the hills and valleys. Noses should be represented as elevated off of the surface of the face. Eyes should be recessed into eye sockets. The human body is not as flat as a pancake, and therefore the naturalistic (or biologically appropriate) image needs to depict every hill and valley in detail.