People who love music, who can lose themselves in a book, or who can spend hours painting a picture of a mountain, know the deep satisfaction that can be found in art. It is not easy to express this satisfaction in words. But, in some partly mysterious (神秘的) way, works of art are among the things of highest value in our lives. A fine piece of music, a masterpiece of painting, or a first-class play has the power to hold our fullest attention. We are completely lost in it, and everything works out right. The music comes to the right close at the right time and in the right way. The play ends, not necessarily on a happy note, but in a way that seems enjoyable. As we get to know more about painting, its parts seem to belong together and to be made for one another. We see harmony (和谐) in the object and feel harmony within ourselves. When the beautiful experience has ended, we often feel uplifted and refreshed. Our eyes and ears, or our understanding of other persons and values, may be sharpened and improved. We may feel more at home with ourselves. Works of art have value for us in some such ways as these. It is this value that marks the difference between great art and simple entertainment(娱乐). A work that is fairly easy to understand takes little effort on our part. It may give us pleasure, but it does not touch our feelings or attract out attention at a deep level. What is the main idea of the text?