Six Principles of Gestalt Theory

Similarity

Similar elements are visually grounded together regardless of proximity

Continuation

Human eye will follow the smoothest path when viewing lines regardless of how the lines are drawn

Closure

The brain's tendency to fill in missing parts of a design or image to create a whole

Proximity

The perception of how close elements are to one another

Figure/Ground

Similar to closure, your brain's ability to distinguish between an image in the foreground and the background

Symmetry and order

Also known as Pragnaz (German for good figure); tendency of the brain to perceive ambiguous shapes in a simples manner as possible