perception
Gestalt principles
The brain's automatic rules for grouping separate visual elements into one perceived whole – designers steer them deliberately rather than fighting them.
1920s perceptual psychology (Wertheimer, Koffka, Köhler) describing how you group things before you consciously think about them. If the brain is going to group elements anyway, a designer’s job is to make sure it groups the right ones.
The four that carry most interface work, in rough order of practical strength: proximity (close together = related), similarity (looks alike = same category), common region (inside one box = one group), and figure/ground (foreground thing vs backdrop). Most “this layout feels confusing” complaints trace back to one of these firing on the wrong grouping.