Gestalt theory is made of 5 principles of visual perception based on the basic principle that all stimuli are perceived as the "sum of its parts." According to Gestalt psychology, the smaller parts of a stimulus are grouped together to form a larger stimulus in its simplest form during perception. It is sometimes referred to as the "Law of Simplicity."
Review paper: Melnick, J. (1997). Welcome to "Gestalt Review": An editorial. Gestalt Review, 1, 1-8.