One of the most compelling contrasts between the ethical beliefs of Plato and Aristotle rests in their arguments on the human behavior. In book One of The Republic, Socrates is trying to prove to Thrasymachus that it is better to be just than unjust. He begins by saying that everything has its own specific function, and that that function is "that which one can do only with it or best with it (Republic 352)." For example, the function of eyes is to see, and since a pruning knife is better suited to pruning than a butcher's knife, its function is to prune. Having established this, Socrates goes on to argue that everything also has a virtue that relates to the performance of its function. The virtue of eyes would be sight and the virtue of the pruning knife would be its sharpness. An object that is deficient in its virtue is said to be incapable of performing its function well (a dull knife would not be able to cut properly). Having shown this, Socrates turns his attention to the human soul and its function. "Is there some function of a soul that you couldn't perform with anything else, for example, taking care of things, ruling, deliberating, and the like? Is there anything other than a soul to which you could rightly assign these, and say that they are its peculiar function? ...What of living? Isn't that a function of the soul? (Republic 353)" Thrasymachus agrees to Socrates' definition of the soul's function and they go on to examine what the virtue of the soul is, that allows it to perform its function. From his previous argument regarding the importance of virtue in the performance of one's function, Socrates extrapolates that a non-virtuous soul would do a poor job of ruling, deliberating, living etc, while a virtuous one would do all of these things admirably. Socrates then references a previous point in the discussion, when he and Thrasymachus had established that justice was the virtue of the soul, and injustice its vice. This allows Socrates to conclude that a just soul and a just man will live well and be happy, while an unjust man will not live well and be unhappy.