The Gestalt principles describe how the visual system groups elements into perceived wholes. The key ones for design:
Proximity: elements close together are perceived as groups. Use spacing to show relationships.
Similarity: elements that look alike are grouped. Use it to reinforce hierarchy.
Continuity: the eye follows lines and curves. Use alignment to create flow.
Closure: the mind completes incomplete shapes. Use it to imply without rendering.
The principles describe perception, not rules. Violating them for emphasis is valid — but you need to understand what you're disrupting.
Related: grids-and-rhythm, ↗ @grace / meaning-through-reference