Pirkei Avot 1:7

"Nittai of Arbel said, distance yourself from a bad neighbor, do not befriend a wicked person, and do not despair of punishment."

After all the previous passages advising us how to live rightly, in accordance with Torah, we come upon this passage that recognizes the problem of evil; people who not only do not attempt to live rightly, but choose, for whatever reason, to act badly.  Nittai recognizes that such people can be all around us: neighbors and people we encounter in daily life.  His advice to distance yourself from them can be contrasted with John Milton's opinion in Areopagitica: "I cannot praise a fugitive and cloistered virtue unexercised and unbreathed, that never sallies out and sees her adversary, but slinks out of the race, where that immortal garland is to run for, not without dust and heat.  Assuredly, we bring not innocence into the world, we bring impurity much rather; that which purifies us is trial, and trial is by what is contrary.  That virtue which is but a youngling in the contemplation of evil, and knows not the utmost that vice promises to her followers, and rejects it, is but a blank virtue, not a pure; her whiteness is but an excremental whiteness; which was the reason why our sage and serious poet Spenser...describing true temperance under the person of Guion, brings him in with his palmer through the cave of Mammon, and the bower of earthly bliss, that he might see and know, and yet abstain."  Nittai's advice also seems contrary to the calling to Jews to be "ohr l'goyim," a light to the nations. 

The opposition of these two views of virtue is truly one of the fundamental issues in the understanding of virtue.  One views virtue as a quality that is developed by study and imitation of the virtuous, because it is a priori derived from Torah.  The second views virtue as a quality which is developed through adversity, and therefore developed a posteriori from experience.  Undoubtedly, the first must be the quintessentially Jewish view, since the commandments of Torah are the only source of virtue, and as we are told in Netzavim, G-d rewards following the commandments and punishes violating them.  The other view bases virtue on the qualities of moral strength of the man; the Latin root of virtue is "vir," or man.  In the Jewish view, the vices of the wicked can be imparted to us if we have excessive contact with them, whereas in Milton's view, contact with the wicked provides "trial," which purifies us.  In the Jewish view, we can be "ohr l'goyim" only by nurturing our yetzer tov, our good inclination, by study and imitation of the virtuous, and then providing an example to others, and G-d provides further motivation to others by exacting retribution on the wicked.