Negative space is the area surrounding the positive subjects of a work of art. The concept is better understood not as a singular aspect of a work, but as a continuous process of interaction between the existent objects in a piece and their environment. Employing negative space into the understanding of art creates a whole new level of visualization once it is fully grasped.
My vision kept blurring as I focused and refocused on my subject: a vase of flowers. I had it propped up on a platform so that it was exactly at eye level while I sat at my easel. I gripped my pencil in my hand, my nose inches from the paper. The intricate details of the leaves and the small flaws in its stem and petals grew more clear to me every moment.
My shoulder was suddenly gripped and my trance was broken.
"What in hell do you think you're doing?" My art teacher gave me a stern glare.
My mouth opened and closed several times. "Um ..."
"Get up." He hauled me off of my perch and planted me several meters away from my easel. "Now," he started, "what do you see?"
I blinked. I tilted my head to the right. I squinted. "Well, ah," I gulped, "nothing."
"That's exactly right. Nothing. Look at your perspective. Your composition. You can't even see your subject from here. Consider both what you're drawing and what you aren't, okay?"
"Okay," I said. I had no specific clue what he was talking about, but I took the criticism directly; I knew how right he was. I couldn't rid it from my mind, and I left school that day feeling dejected.
Later that afternoon, I left my constricting house for a walk. It ended as I plopped onto a bench outside of a large brick building. I noticed for the first time that the bench was positioned directly between two large oak trees. My curious gaze followed the trunk of one tree all the way up its base until it separated off into dozens of splintering branches that reached over my head. There, the branches criss-crossed with limbs from the tree on my other side. My mouth sagged open in wonder; the autumn season left the leaves from one tree a vibrant crimson, while the other leaves were glittering yellow. They formed endless patterns with the brilliant evening blue sky; brown branch sliced across red and yellow, which were layered over one another in an elaborate cloaking tapestry over my head. The bits of blue filtered through, caressing and bathing the whole glorious sight in mellow, gentle sunlight.
The effect was devestating; finally I saw what I had been missing. I saw how unifying negative space was. The trees would lose all of their perfect form and structure without the background of the sky and their ever-shifting interaction with it.
I lept up from the bench in a flash of inspiration.
The negative space in a work is not necessarily empty. It constitutes any portion of a work that can be visualized as separate from any subject in that work. One of my favorite artists, M.C. Escher, is probably one of the best to study in terms of this concept.
Look, for example, at his piece "Three Worlds"1.
Observe the three main things Escher is working with here; there is a surface plane of water that is covered in leaves that extends into the distance to a horizon that isn't visible to the viewer. There is also a large, shadowy image of a fish swimming underneath that plane of water, and two shadows of trees. Therefore, simply by interaction between the plane of leaves and the image of trees, he has to convey the fact that the trees represent a shadow of two actual objects.
In doing so, in terms of negative space interaction, he has to consider the spacing between the branches themselves and how that contributes to the overall shape and construction of the tree: Is his consideration of the space between and surrounding each element of this object appropriate for an accurate representation of a tree? This same consideration must also be applied to a factor of depth; the tree has to look like it is vanishing deeper into the water from the top of the image to the bottom; spacing between each branch and limb should grow smaller.
The same process must be applied to the leaves. Rather than a structured approach to negative space among these objects, scattered leaves will appear more haphazard in their arrangement. Therefore, their random, layered appearance stands in contrast to the solid, evenly spaced branches of the tree shadows, creating, simply by negative space considerations, a sense of depth, which is central to what Escher is attempting here.
Stand back from a piece of art. Observe how the elements of foreground, middleground, and background interact within it. Look at the types of shapes created in a piece. Are they curving or angular? How does this contribute to the overall composition and feel of the work? If you can learn to ask these questions of a piece of art, you will learn to see a whole new dimension to it, and eventually that will broaden to a new way of seeing the world.
1"Three Worlds." M.C. Escher. © 1955. - Online copy not available right now.
Sourced mainly from personal experience.
Further Reading:
http://home.att.net/~tisone/lesson15negative.htm — contatins a short drawing exercise involving positive/negative space.
http://www.fionabroome.com/fae/posnegspace.htm — this comes from a Wiccan website. The negative space visualization process she uses is quite interesting.
Look at any of Escher's tessellations for further examples of this concept. | 
