Saying everything completely leaves nothing more to be said. 

A painting that allows its viewers to fill in missing details can be more alluring than one that meticulously describes every single element.  One effective method for achieving this is the lost edge.

A lost edge is an area of a shape not defined, but merged into the shape or background beside it.  Look at these two examples:

Compare the right side of the bird's head and the left side of the neck in both renditions. The image labeled A is isolated from the background because all edges are clearly defined whereas in B, the edges of the head and neck area merge into the background, a use of lost edges allowing the bird image to unify with its surroundings.  

Here's the complete painting.  

Tip:  Any time a shape and its surroundings are in the same value range, there is an opportunity for the shape's edges to disappear or fade into adjacent areas rather than be delineated.  To do so helps unify the whole piece.

In the two photos below, the drawing on the left is completely delineated. On the right, I have taken away the lines where the lights of the image meet the light background.  In this version, we still read the drawing as that of a dog, but it becomes more interesting as a drawing.

It is the lost edges in the painting below that make it work, that cause the images of the leaves to feel merged with the surrounding woods and trees thus allowing the eye to freely roam within the painting.

Here's an analysis that shows a few specific areas where lost edges actually keep the eye moving throughout the painting.

Dianne