Why it is interesting what Glaucon states that all goods can be divided into three classes: things that we desire only for their consequences, such as physical training and medical treatment; things that we desire only for their own sake, such as joy; and, the highest class, things we desire both for their own sake and for what we get from them, such as knowledge, sight, and health. What Glaucon and the rest would like Socrates to prove is that justice is not only desirable, but that it belongs to the highest class of desirable things: those desired both for their own sake and their consequences.