For a long time, people drewa clear linebetween good and evil - the good guys were perfect, while the evil guys were all nasty.
长久以来,人们把好与坏分的泾渭分明——好人完美无缺;坏人一无是处。
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I think one of the most important meanings is that it drewa clear linebetween the waiter who "provides service" and the guests who are "served" by the waiter.