Plato arrives at his definition of justice through his and Socrates' analysis of human nature. Plato believes that humans are born unequal - some are born stronger and some wiser, for which reason the wiser ones know what is best for the city and become the rulers. The ruler must have four virtues which are essential for a good governor: wisdom, courage, moderation, and justice. In order to arrive at these virtues Plato uses the method of dialectic, the asking and answering of questions which lead from one point to another, with supposedly irrefutable logic by obtaining agreement to each point before proceeding on to the next, and so building an arguement. According to Plato, goodness means doing what is best for the common, greater good rather than for individual happiness, so ultimately good, or virtue, is its own reward. He ties excellence to one's function, comparing it to the body and its senses and how each organ is considered excellent if it performs its function well, like eyes to see or ears to hear. Of the four virtues Plato identifies, justice is perhaps the most important one and it resides not only within man as an individual, but in man's relations and interactions with other men. So he defines a happy person as one who is just and performs his function well; like the organs of the body, man's virtues work together, performing their respective functions and contributing to the whole. Justice is therefore the greatest virtue of man, for it is crucial in making the other virtues work together for the common good like the organs combine for the good of the entire body. So justice is thus an excellence in social organization and in the organization of the human soul. Like the organs in the body, all contribute to the whole, but the eyes only see, the ears only year, and they do not share functions. Using this analogy, justice is like the moral mind which guides the body in its activities; it's at the head, at the tope of the hierarchy in social terms.
