The fifth rule of perception  is figure-ground perception.  This is when one distinguishes a certain object (the figure) from other objects (the ground or background).  This rule can be seen in the pictures below.  In the first picture, the book page cannot be read because the focus is on the ground (the stool) instead of on the figure (the book page). In the second picture, the book page is clear, and the stool it sits on is blurry.  This is because the focus is on the figure and not on the ground, as opposed to the first picture, where the opposite is true.