Another principle of Gestalt is CLOSURE. For closure, the mind supplies the missing pieces to "close" a form, composition, etc.
In the image below, there is no actual lightbulb. The mind wants to close the shape formed by the hand while the hardware at the bottom aids in the suggestion of "lightbulb".
The next well-known logo for World Wildlife Federation uses closure to create a panda bear.
The following are part of a designer's "Gestalphabet" where he uses human behaviorism to "close" the open forms in order to create a recognizable image. The viewer then sees A, B, C, and D when he has not actually used any of these letterforms.
Closure is often used in logos. The S in USA is formed by closure. The girls' faces in the Girl Scouts are formed by closure and Figure-Ground.