In the Form Principle Loomis tried to bring together all factors involving line, tone, and colour. He said…
[The first step in creating an image is to determine] what kind of light we are working with, for its nature and quality and the direction from which it comes will affect the entire appearance of the form… All forms within your picture should appear to be lighted by the same source and be lighted consistently with one another.
Light is the most important factor in determining the appearance of form.
Small forms (such as folds of cloth) should be subordinated to the big form – only draw surface forms that give expression to the big form.
The best pictures run to a few simple values
The design makes the picture, not the subject or material
Light and shadow in itself can produce design.
You cannot avoid making your subject either a tonal statement or a linear statement: All pictures are fundamentally either arrangements of lights, intervening tones, and darks, or else linear arrangements (though a picture can combine both).