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