Romeo and Juliet 

Friar Laurence is going around picking herbs and flowers that he can use in the making of his medicine. He questions how a flower that can be both good and evil in lines 4 and 5 of this passage, "For this, being smelt, with that part cheers each part; Being tasted, slays all senses with the heart." Literally, the friar is talking about how a flower possesses the ability to both cure and kill.

In the next two lines, Friar says "Two such opposed kings encamp them still in man as well as herbs, grace and rude will;" Now Friar Laurence extends his metaphor to mankind, saying that two opposites also dwell within the soul. He makes a comparison between a man and a plant in saying that both are capable of good or evil. 

In the next lines, Friar Laurence warns that when evil becomes more prominent than the good in a man, death eats up the plant. Once it was said that within a man's soul are two dogs fighting, one good and one evil. The dog that wins is the one the man feeds. This relates to the passage in that both suggest that men are equally good and evil at birth, and the man's choices will determine which is more predominant in the person's personality. Shakespeare goes further to say that if evil begins rearing its ugly head then it is only a matter of time before a person is lost to evil doings.

A common debate throughout history is whether human nature is mostly good or mostly evil. It is in breif passages like this that Shakespeare shows his brilliance. He aims to show his opinion of human nature in 8 lines of literature, the likes of which can easiily be passed over by a reader at first glance. But upon further inspection, all of Shakespeare's literature is constantly posing questions and possible answers about universal problems.