2. Our Vision is Optimized to See Structure

Gestalt principles

Proximity
Objects relatively near each other appear grouped. ex) spacing for radio button and group.

Similarity
Objects that look similar appear grouped. Shape, size, color, and other information. ex) complex menu, icon-only buttons and text-only buttons look grouped separately.

Continuity
When humans perceive information, they tend to connect, fill, and update the information when it's ambiguous. ex) IBM logo. X looks like a two line crossing each other rather than two horizontal/vertical chevrons.

Closure
Humans perceive "unfinished looking" objects and try to give it a "closure". ex) WWF logo. Who's that pokemon meme.

Simplicity (Symmetry, Good Figure)
Humans tend to interpret complex objects the simplest way possible first even though it has multiple ways to desconstruct. ex) Olympic logo. Stacked papers. Table. Chart.

Foreground and Background (Figure and Ground)
When multiple objects overlap, humans usually think smaller objects as figure and larger ones as background. ex) your phone home screen (App icons and background)
The viewer's focus of attention can alter this. ex) old woman facing forward or young woman turning her head? 

Common Fate
Objects that move together can be looked grouped even though they are not different that static objects. ex) several people walking in the same direction and speed. Street racers weave around cars at a similar speed.

Combined
You can check your design by applying each principle individually and see if there are any unintended principles.