The Tale of the Ring of Gyges

Suppose now that there were two magic rings, and the man of justice put on one of them and the man of unjustice the other; the man of justice cannot be imagined to be of such an iron will that he would stand firm in justice. No man could keep his hands off what was not his own when he could safely take what he liked out of the market, or go into houses and lie with any one at his pleasure, or kill or release from prison whom he would, and in all respects be like a God among men. Then the actions of the man of justice would be the same as the actions of the man of unjustice; they would both come at last to the same point.

And this we may truly affirm to be a great proof that mankind is just, not willingly or because he thinks that justice is any good to him individually, but of necessity, for wherever any one thinks that he can safely be unjust, there he is unjust. For all men believe in their hearts that injustice is far more profitable to the individual than justice, and he who argues as I have been supposing, will say that they are right. If you could imagine any one obtaining this power of becoming invisible, and never doing any wrong or touching what was another's, he would be thought by the onlookers to be a fool. Although, they would praise him to one another's faces, and keep up the appearance of praise with one another, out of fear that they might be the first to suffer the inevitable injustice.

--From Plato's Republic, Book 2