Ultimately, the Good is the causal element of justice, but in order for the state to become just someone must first come to understand the Good and consequently justice as well. According to Plato, the sole possessors of this particular knowledge are the philosopher kings, who are either kings made philosophers or philosophers turned kings. Their ability to understand the good is initially cultivated by their ordained education which focuses on the study of subjects that will lead to the discovery of truth and being, namely the mathematical sciences and dialectic.
Once their education is complete, they will not only be able to distinguish between the images of the visible world and the forms of the intelligible world, but most significantly will ultimately come to know the Good. Once known, the philosopher kings must act under its influence and order both their soul and the state in the likeness of its eternal model. 8 This is how in Plato's Republic his archetypal city-state becomes just, virtuous and good and how the Good becomes substantive within the visible world creating an underlying philosophy of order.
The Good, in conclusion, is fundamental as it creates the eternal forms, allows them to be grasped by mind, compels the ordering of the state in its representation and engenders justice in the process by those who have come to know it. Since the Good itself is explicitly considered to be beyond the knowledge of the interlocutors the investigation is re-directed specifically to examine the behavior of the Good. Socrates uses the sun as a demonstrative device, likening it to the Good and then showing how each functions respectively within its own realm.
He discusses the innate goodness of the rulers of the state and maintains that by their decisive ordering of society it will in turn function under the principles of justice, virtue and above all the Good. So this Platonic work does not simply address the issue of justice, as was originally intended, but extends to touch on something even more fundamental. However, the Republic is a mere introduction to this expansive subject and is not intended to investigate the Good beyond its relationship to the form of justice and its role in the establishment of a just state.