Forms that enter the plane have three coordinates— distance and direction from the abscissa, distance and direction from the ordinate, and distance above or below the plane. A canvas is physically bounded, but it displays forms limited only by the spiritual functions that generate them. Ideal dematerialized elements free expressions and determine how they fit on the plane.
We have a natural sense of up and down, of left and right, and of front and back. These are our compositional tools, to place ourselves and to tease our limits.
Gravity, heat, and a sense of importance pull us down, toward the warmth, and forward. Action and reaction balance to create tension. Every physical force has a corresponding ideal that drives it.
We try to manage space, to make the inside bigger than the outside. The third dimension is always present, and no good artist wants to fake it. Some try for a totally flat effect, but depth may remain in a gradation of color or in objects of different sizes. The boundaries of the canvas may suggest depth, like an open window, or a surface of a lake of unknown depth.
Progression of color or size, traditional perspective, impinging, overlapping, or interpenetrating planes. Many factors combine. Inner necessity demands depth and compels us to plumb its mystery.
Unconsciously, we complete figures, assume they have depth, and animate them. We relate figures to each other, and connect foreground and background. The artist works with us or against us.
The eye moves from light to dark or vice versa, creating inside and outside, forward and backward. Contrast creates energy, with shading impinging on lines suggesting bas-relief or high relief. Distance is measured from object to object; space is measured from the center. However, whether we see a vase or two faces in profile may be a problem.
Stage left is right; stage right is left. I am right handed but the actor in the mirror is a lefty.
In a garden, for example, the unique is articulated; it moves. Something happens in this space, a deviation from expectations, demanding attention. If the third dimension weren’t happening, then you wouldn’t be probing for meaning and assuming the actors were performing for you.