This is the second lesson in the Art of Visual Expression Basic class
The artist/painter is like the magician, creating illusions on a flat surface. It is important to remember this because it is easy to get caught up in representation rather than expressing an experience in paint. Magritte's "This Is Not A Pipe" is a wonderful reminder to be conscious of the canvas and paint (or paper and graphite) as physical realities which have been manipulated to give the illusion of something else, (or not as in the case of pure abstract art). Succeeding in expressing in drawing format what you see in reality comes down to learning about the medium and its limitations, and then, as you develop an intimate knowledge of this (ie practice), the gap between what you see and what your expectations are of the drawing narrows until you reach that marvelous achievement of artistic integrity.
Lines on paper create illusion. It takes only a few simple strokes of a pencil to give the sense of depth on a flat plane. The diagonal and the horizon line are two of the most powerful lines in any representational composition because our mind interprets these movements across the page in a very specific way. It is actually very difficult to not imply depth in a painting if these lines are present. Because we humans are conditioned to interpret these specific types of lines as depth, once understood, it becomes quite easy to trick the eye and create the illusion of objects in space all on a flat piece of paper....magic.