People have different attitudes towards living. Some seek self-oriented happiness, without any consideration for others. To them, "happiness" means
wielding political or economic power over others. But most of these people turn out to be disappointed in their pursuit of happiness: since man's desire is endless, nothing can fully satisfy their needs.

Other people believe that first step to attaining happiness is to
restrict personal desire. Priests, monks, and nuns are the most common representatives of this group. By refraining from worldly passion, they think they are happy because they live their lives in accordance with religious commands. But, can anyone without personal autonomy be genuinely happy?

A third group consists of very ordinary people. They have no great wealth or political authority. While they may have religious beliefs, they do not turn their back on the world. But they treasure their families, they value the little things of everyday life. They are happy if they have three meals a day, a car, a home, a vacation, enough money to educate their children...

Which group of people is the happiest? Who knows? If you think you are happy, then you are. Happiness is the result of how you think about the world and your place in it.

問題：turn one's back on 和Who knows?什么意思，怎么理解