In Book II of Plato's Republic, Glaucon and Socrates present two contrasting conceptions of human nature. For Glaucon, mankind inherently tends toward evil. Everyone would commit crimes if they could do so without any consequences. For Socrates, however, humankind is merely corruptible. Without external evil influences, each a person would choose what is right. These two views become apparent in the story of the ring of Gyges and in the discussion of the guardians' education.
Glaucon tells the story of the ring of Gyges in order to show that everyone would commit atrocities if they could do so with impunity. Glaucon says, "Let's suppose, then, that there were two such rings, one worn by a just and the other by an unjust person. Now, no one, it seems, would be so incorruptible that he would stay on the path of justice" (Grube, 36). Glaucon thinks that humans choose to act ethically only because they fear the consequences of acting immorally. In this view, man is naturally inclined to do evil.
Socrates, however, thinks that humans are good so long as they are not exposed to evil influences. Concerning myths about the gods doing immoral things, he says, "The young can't distinguish what is allegorical from what isn't, and the opinions they absorb at that age are hard to erase and apt to become unalterable. For these reasons, then, we should probably take the utmost care to insure that the first stories they hear about virtue are the best ones for them to hear" (Grube, 54). For Socrates, humankind has the capacity for corruption, but it is possible that this capacity never actualizes. If people are not exposed to wrongheaded thinking or to immoral myths, then they may do what is right. In this view, humans are neither inclined to good or evil. Instead, their environment influences them.
Thus Glaucon and Socrates hold distinctly different views of human nature. One posits that man is naturally inclined to act unethically. In this conception, people only ever act virtuously because they fear the consequences of acting immorally. The other maintains that people have the capacity for both good and evil. In this conception, environmental or circumstantial influences play a significant role in forming a virtuous agent. These views entail certain consequences for political philosophy, and Plato's kallipolis presupposes Socrates' view. If Glaucon's conception is correct, then Plato's just city will never be more than a utopian dream.