Gestalt psychology

Gestalt psychology basically says this: people are smart. They like to figure things out. They don't want things to be laid out so plain and simple that there is no challenge. So, when you apply this to art, this means people like to figure out what the piece means, or is supposed to be, etc. There are several parts to Gestalt psychology:

Emergence: we can form entire images when just a small part of the image is shown and the rest is implied
Reification: we can form shapes with implied and imaginary lines
Multistability: when an image can pop back and forth between two different images/objects
Invariance: we can recognize geometric shapes even when they are rotated or morphed
Pragnanz: we like to order things

• law of closure: if you draw something with a broken line, we will just visually close the line
• law of similarity: our mind groups things with similarities
• law of proximity: our mind groups things according to distance
• law of symmetry: things that are symmetrical are groups regardless of distance or location
• law of continuity: our mind will continue with already formed patterns
  

• law of common fate: our mind groups things that are moving in the same general direction

These rules and theories are very important to keep in mind when making art because we don't want our viewer to think that we, as the artist, think the viewer is dumb. We want to give the viewer the satisfaction of "figuring it out" after looking at our work. And, sometimes, leaving things out or making things harder to figure out make the piece more interesting and unique.