… Only incidentally does the spatial quality derive from the fact that optical signs resemble objects known empirically.
This is from Language of Vision by Gyorgy Kepes (1944):
… The forces of visual attraction — a point, a line, an area — exist in an optical background and act on the optical field. This optical field is projected on the retinal surface of the eyes as an inseparable background for the distinct visual units. One cannot therefore perceive visual units as isolated entities, but [only as] relationships. “As so called optical illusions show, we do not see individual fractions of a thing; instead, the mode of appearance of each part depends not only upon the stimulation arising at that point but upon the conditions prevailing at other points as well” [W. Kohler].
Color and value depend always upon the immediate surrounding surfaces. A brightness value can be amplified or blotted out by the other values. A color can be intensified or neutralized in the same way.
The same is true of texture qualities. Sizes and shapes likewise are perceived only in polar unity with a background and their specific optical quality is due to their respective frames of reference. A slightly irregular shape appears strongly irregular in a frame of reference of geometrically perfect squares, but the same shape appears perfectly regular in reference to extremely irregular units. Generally speaking, all the optical units on a picture surface derive their qualities in relationship to their respective backgrounds, ranging from the immediate surrounding surface to the optical field as a whole.
… Since each shape, color, value, texture, direction, and position produces a different quality of experience there must arise an inherent contradiction from their being on the same flat surface. This contradiction can be resolved only as they have the appearance of movement in the picture plane. These virtual movements of optical qualities will mould and form the picture space, thus acting as spatial forces. Only incidentally does the spatial quality derive from the fact that optical signs resemble objects known empirically.
… In the diagram taken from Kopferman, the black squares in a rectangular outline which indicates the boundaries of the picture plane demonstrate the modifications of the same shape under various conditions. Wherever the small square can be brought into accordance with the main direction of space it is seen as a square, partly because it is parallel with the borders of the picture plane, and partly because it is actually in a horizontal-vertical position in regard to the next frame of reference — the page [or, in our case, the screen]. It is thus dependent upon the ground on which it appears.
[line break added] If the ground has a definite correspondence to the horizontal-vertical axis, however, the square figure in a diagonal position not only loses its stability but undergoes a modification. It is seen, not as a square, but as a diamond. A study of the diagram makes it obvious that the relationship of the unit to the picture border generates its spatial expression. In one case it appears static and suspended; in another, static but with strong resistance — almost with a quality of solidity; in a third case, it changes shape and loses its concreteness; finally, it suggests a potential movement and fluctuates between the square and the diamond shape.
… Just as the letters of the alphabet can be put together in innumerable ways to form words which convey meanings, so the optical measures and qualities can be brought together in innumerable ways, and each particular relationship generates a different sensation of space. The variations to be achieved are endless. For while the elementary signs of the English language are only twenty-six, the number of elementary forces with which the machinery of sight is provided is prodigious.