名詞 beholder:

1. a person who becomes aware (of things or events) through the senses
   同義詞：perceiver, percipient, observer

1. I don't understand how he can think she is good looking, but beauty is in the eye of the beholder.
   我不明白他怎么會認為她漂亮，不過情人眼里出西施罷了。
2. She seems very plain but he dotes on her, but beauty is in the eye of the beholder.
   她看上去相貌平平，但他非常愛她，情人眼中出西施。