Protagoras by Plato

This is a dialogue between Socrates and Protagoras which is wide ranging in scope, but centers on the virtues of justice and wisdom.  Protagoras is a sophist who is pretty darn confident that he can make any one who studies with him into a good citizen.  Socrates is not a big fan of the sophists, and the thinking is that it is because they do not choose their students, their students choose them.  For the sophists, cash is king and aptitude comes a distant second, whereas Socrates is more of a fan of the bright and talented student rather than the wealthy student.

Socrates method of dialogue or philosophical inquiry is called the elenchus.  Essentially the person engaged in the dialogue with Socrates, in this case Protagoras, asserts a thesis, and in the case of Protagoras, he starts with the assertion that he can make anyone he teaches a good citizen.  Socrates asks a number of clarifying questions and then moves to securing the agreement to a second premise, one that is linked to the first.  Socrates then argues, and works to get the other person to agree, that what this further premise, or group of premises imply end up being in contradiction to the original thesis.  Socrates then claims that he has demonstrated that the persons original statement or thesis is false.  He is kind of annoying about it, but the progression of the dialogue is in and of itself quite fascinating.