The Tragedy of Romeo and Juliet: Act 2What paradox does the friar identify in lines 15 - 22?
The Friar identifies a universal paradox in the line "naught so vile that on the earth doth live/But to the earth some special good doth give." That is, there is nothing on earth that is either wholly terrible or wholly evil, because everything contributes its own particular thing to the balance of life. The Friar goes on to say that there is also nothing on earth that is so perfectly good that it does not, given the right motivations, have the capacity for "abuse."
The couplet which concludes this section (line 22) encapsulates the Friar's point: essentially, "virtue" can become "vice" if it is used incorrectly; likewise, sometimes bad things can be done in pursuit of a noble purpose, which turns "vice" into "action dignified."
The Friar is here talking about the flowers he uses which, outwardly "infant" or weak, actually contain poison strong enough to kill a person. However, more broadly, his comments refer to the behavior of the actors in the play. More generally still, the Friar is discussing the wider human condition. Romeo, in listening to him, interprets the Friar's words as suggesting that if "bad" actions can be somehow justified, then they are no longer bad. Obviously, interpretation of what is "bad" is wholly subjective.
The friar is gathering herbs and plants that can be beneficial to people, but then he recognizes that the same plants and herbs can be deadly, as well, when used inappropriately. The friar realizes that the same is true for man. Both herbs and man contain the ability for good and evil.
The friar experiences this first hand. In attempting to do something good-agreeing to secretly marry the young lovers, his actions without considering dire consequences lead to Romeo and Juliet's destruction.
This is especially ironic since he cautions Romeo to "Wisely and slow-they stumble that run fast"(II, iii) and he is guilty of ignoring his own advice.
The friar is like a plant or herb, he is in many ways a resource, yet his actions lead to dire consequences for those who rely on him.