Visual unity is one of the most important aspects of well-developed art and is planned by the artist.
Unity provides the cohesive quailty that makes an artwork feel complete and finished.
When all the elements in a work look as though they belong together, the artist has achieved unity.
Unity in Fine Art
Van Gogh was concerned with the unity of his paintings. In this one, the swirling brush strokes and dominance of cool colors tends to unify the surface and create the feeling that everything belongs together.
Source: Elements and Principles of Design: Student Guide with Activities, published by Crystal Productions