Socrates begins the argument on the basis of a statement from a past concession that "just people are clever and more capable of doing things, while unjust ones are not even able to act together." (351b). The purpose of this argument is to determine whether or not just people live better and happier than unjust ones. Socrates starts off by refuting Thrasymachus with such question, that just people do live better lives, and that it is quite obvious. The whole argument is important to Socrates because he feels that it is concerned with the way people ought to live. .
Socrates than questions Thrasymachus about functions of certain things, and we are given the impression from Socrates that everything has its own specific purpose, and does it well. To explain this Socrates gives the example about pruning vines, and states that any knife would do the job but obviously a pruning knife would be the most successful. With this example he states more clearly that a function of something can act alone for what it is capable of or what it does better than anything else. (353a).
Thrasymachus understands this and once again agrees with Socrates" reasonings. Socrates continues in asking whether a particular function has a virtue. Thrasymachus agrees only to have Socrates state that: a function performs well by means of its virtue and badly by the mean of its vice. Thrasymachus continues to agree, while Socrates states that if a function is deprived of its virtue, it could only perform poorly. To prove this argument, Socrates uses simple concrete examples of the function of eyes and ears, which both constitute their own virtue and vice. If you take away eyesight you are left with blindness, sight being the virtue and blindness being the vice. From here we have concluded that anything that has a function also has an excellence. When putting them together one can assume that everything performs its own function only if it has its similar excellence (its virtue) and not it's evil (its vice).