Example on this page from
The Perceptual World, and
The Optical Illusion Book.
(See Credits)
   The Law of Pragnanz, also known as the Law of Simplicity, states that we tend to perceive the world in the simplest form possible.  There are two major parts to this law: closure and grouping.  Closure is the tendency to fill in missing portions from a perceptual array, or to fill in the gaps.  Grouping occurs when similar or proximal stimuli are perceived as belonging together in a group.  
     A common example used to illustrate closure is the perception of a complete triangle from three lines.  This can be obseved at right.  If you look closely you can actually see two triangles in the figure.  Grouping can be observed by looking at the collection of black dots below.  Your brain groups the dots together and allows you to see the figure of a dog.