From a very early age, we develop perceptions of how things are “supposed” to look, and, as children, we create symbols to illustrate these visual perceptions. Common subjects include people, faces, trees, and houses. Each attempt is exactly the same; people, for example, are depicted with a circular head, two dots for the eyes and nose, and a single line for the mouth. Other symbols are used to depict the limbs, hands, feet, etc. Only when we train ourselves to really look at these subjects, do we learn how to actually “see” what is there.
One very common mis-conception is that bold lines must be used to distinguish the separation between object and background. It’s not that lines don’t play a significant role in the drawing process, but they should be used sensitively and selectively. If they are over-used, they will flatten out the appearance of form.
To create a realistic drawing, one must first become accustomed to seeing value instead of line. The illustration below displays a very simple grey scale, and a demonstration of it’s proper uses in a basic tonal rendering. Square #1 is achieved simply by leaving the white of the paper. Shading was added around the edges to demonstrate that a light area can only stand out if it’s surrounded by a darker value. Square #3 is a mid-tone, (the value that falls between the lightest light, and the darkest dark) and square #5 is the darkest value that could be achieved with a pencil. Values 2 and 4 were added last and fall half-way between the values of the squares bordering them. Keep in mind that this is a very simplified grey scale — obviously the amount of greys that the human eye detects is far greater than the range displayed here.
When applying these values to a representational drawing, they should, at first, be simplified as much as possible. Example A only uses values 1 and 2 and yet there is still enough range to show where the light and shadow falls. Example B introduces value #3 which brings out a little more form. Example C is where values 4 and 5 get added, and more shading transitions become apparent. Notice that, as more values get added to this drawing, the lines become more obscure and eventually they are eliminated all together.
Riveradavid's Blog 
Yes!
My Zen Buddhists friends talk about form and emptiness or something like that–we have a mental tendency to perceive a substantial boundary at the interface “between” visually distinct regions. “Between” — ugh — that’s not a great word. An interface is not “between” two regions; we identify interfaces where two regions meet because nothing else is there.
More generally I was telling my friend about “unseeing.” I told her to notice in real life the near absence of lines and pure white (most “white” things I see in daily life are tinted). She had a psychedelic moment.
Absolutely David – very rarely will a “white” object actually appear as such. The human eye is capable of detecting thousands of grey shades. This is especially imortant to remember when rendering objects with color – every color relates to a specific value.
Thanks for the comment!