For many people the difficulty in creating art arises not in the mechanics of putting pencil to paper or brush to canvas. The techniques by which art is made can be easily mastered with sufficient practice. Still, the result may not live up to what the artist expects. This may be because the person is practicing the wrong thing, at least in part.
Every scene is composed of small parts which stand in some relationship to each other.
The normal approach to this reality is to ignore most of the relationships. Some are simply more important than others most of the time. However, when attempting to render a scene accurately, every relationship becomes important. For example, consider to glasses standing on a table. If one's goal is to get a drink it is important to note which glass is closer or to the right or left of the other.