In Plato's Republic, Socrates and the interlocutors Polymarchus, Thrasymacus, Adeimantus, or Glaucon discuss the nature of justice. They investigate many ideas pertaining to what justice is, outline the various sections of society, and eventually come to define justice itself. The climax of the dialogue is reached in Book VI when the underlying cause of justice is revealed. This fundamental principle is not limited solely to justice, however, but is the cause of all of the virtues.
Socrates utilizes an analogy of the sun, as it pertains to the sensible world, to illuminate the way in which the Good, the underlying principle in question, affects the intelligible world. He demonstrates how the ideal rulers come to know the Good by way of their education and that once this knowledge is possessed they can implement order that will bring about a just, virtuous and ultimately good city-state. The Good causes the forms of the intelligible world and makes them understandable to the intellect, and in doing so creates justice both within the soul and the state.
Before any kind of exploration of the Good is attempted two ideas must be made clear. Firstly, Socrates does not ever explicitly say what the Good actually is. In fact, he states that it is too wide and complex a subject and will therefore leave it unexamined. 1 Consequently, he elucidates what he calls an 'offspring of the Good', which is an analogy of the nature of the sun as he will show it to relate to the nature of the Good. The Good is only important in this dialogue in so far as it concerns the nature of justice, and is not meant to be a topic of enquiry in and of itself.
This is why the analogue is sufficient and defining the Good is not crucial to the argument of the dialogue. This also, being an excellent demonstration of Plato molding the souls of the interlocutors through his "philosophy-at-work" type teachings. Secondly, Plato describes two types of reality: the visible and the intelligible. It is important to distinguish between these two realms, in order to understand the difference between the images of the visible world and the forms of the intelligible world, as they pertain to the sun and the Good.
The visible world is transitory, changing and always becoming. It contains the substantial images that are perceptible to the senses. Conversely, the intelligible world is timeless, unchanging and always being. It is not available to the senses but rather exists within the confines of the intellect. The images of the visible world are made in the likeness of the forms of the intelligible world, "therefore, say that not only being known is present in the things known as a consequence of the good"2 by what Plato calls participation.
Trees, for example, come in many different types but the reason why they are all called a tree and not a shoe or a cucumber is because each participates in the universal form of 'tree'. What this means is that the form of 'tree' exists within the intellect and is the means that allows one to judge the extent to which the image is tree-like or shoe-like or cucumber-like. That participation in the form is what makes it a tree and not a shoe or cucumber.
Furthermore, there is another category of form. This type subsists within the mind but is not perceptible to the senses, in that it can not be smelt or touched or tasted. These forms relate to ideas such as justice, beauty, truth, etc. , and are what were called virtue by the Ancient Greeks. Understanding the difference between these two realms is essential in order to distinguish between something which is good (or participates in the Good, to use Platonic language) and what is the Good itself.
Socrates explains to Glaucon in the dialog that, "although the good isn't being but is still beyond being, exceeding it in dignity and power"3 Glaucon is quite perplexed by Socrates' opinions and estimations of what the Good truly is, he said in refute to Socrates; "Apollo, what a demonic excess"4. Socrates uses the simile of the sun specifically in order to show the nature of the Good. He states that eyes are constructed to see just as the images of the visible world have colour in order to be seen.
However, without light to illuminate the image to the eyes, one will remain unknown to the other and a state of perpetual darkness will abound. Therefore, it is the sun that illuminates the image to the eyes and allows it to be seen. In the case of the Good, the mind is constructed to understand just as the forms of the intelligible world are constructed to be understood and grasped by the mind. As the sun illuminates the images of the visible world, so the Good illuminates the forms of the intelligible world. Understanding this relationship is the first step to understanding the Good.
Furthermore, the sun not only makes the image visible to the eye, but more significantly causes the image itself. The sun accomplishes this by giving the image nourishment, warmth and everything else necessary for it to grow, although the sun itself does not change or develop. It allows the image to come into becoming. It may seem relatively clear that the sun causes the images of the visible world because one is familiar with the nature of the sun and its relationship to those images. It is, however, somewhat less evident how the Good causes the forms to come into being, as the nature of the Good is still rather elusive.
As Socrates does not say exactly what the Good is he can not describe how exactly it causes the forms and as a result must rely on the sun imagery to explain this. Even when he is contemplating the imagery of the sun with Glaucon, Socrates discusses that when exploring it, "I am leaving out a throng of things"5 that could be pertinent to developing ones own opinions on the subject. In the same way that the sun allows the images to be seen by the eye, so the Good allows the forms to be understood by the mind. Moreover, as the sun illuminates those images it also causes them to come into existence.
Similarly, as the Good allows the forms to be understood, this understanding or knowing makes them substantial within the mind and allows them to come into being within that intelligible realm. The Good causes the forms, but at the same time is transcendent of them. Socrates' explanation, then, is simply that the Good makes the forms knowable, but is also their cause. 6 There is one more way in which this simile is further extended. Socrates explains that just as sight and light are sun-like but not the sun, so qualities such as truth and wisdom are good-like but not the Good.
7 In the preceding paragraph it was briefly mentioned that both the sun and the Good cause either the image or the form to come to fruition, but that they themselves do not undergo any sort of change. This is indicative of a transcendence that separates that which is caused from that which causes. This is important to realize because it shows a kind of hierarchical structure. Within the intelligible realm it is particularly significant, as knowing the Good means understanding virtue (the second kind of form discussed earlier), of which justice is one form.
It would seem, then, that saying a just man, a virtuous man and a good man is in fact saying the same thing three times. In showing this, it demonstrates the inextricable connection between the Good and justice. It is for this purpose that the Good is included in the Republic, because without it justice can not be known and if justice can not be known than the original question of 'what is justice? ' can never be answered. Therefore, to know the Good is to know justice.