The Gestalt psychologists of early 20th-century Germany — Wertheimer, Köhler, Koffka — observed something fundamental about human perception: we do not see the world as a collection of independent stimuli. We see patterns, groups, wholes. The mind imposes structure on visual input, filling gaps, completing shapes, and grouping elements by shared properties. Design that understands this works with perception; design that ignores it works against it.
Proximity is perhaps the most powerful Gestalt principle for UI designers: elements that are close together are perceived as related. This means that spacing is not decoration — it is information. Moving two elements closer together tells the user they are connected. Moving them apart tells the user they are distinct. Every spacing decision in a UI is implicitly a Gestalt decision.
Closure — the tendency to perceive incomplete shapes as complete — explains why icon design works. A circle with a gap is still read as a circle. A partially occluded rectangle is still read as a rectangle. Interface designers exploit closure constantly: progress rings, notification badges, cropped images in cards. The mind completes what the eye only partly sees.