"One of the things that spoil most people's drawings is the appearance of flatness, or two- dimensionality. Height and width are very important, but depth is the mark of the master artist.
The question is: "how do we make it look like the real thing and not some flat one dimensional graphic on the page?" How do we make the viewer feel that he or she could reach out and touch it? In short, how do we give it the proper form?"