You have often heard it said that the Form of the Good is the greatest thing to learn about, and that it is by their relation to it that just things and [other virtuous things] become useful and beneficial (Republic, 505a).
Plato’s Republic is a wide-ranging tract, admired for its depth, nuance, and ambition. Plato sets himself to answering two questions: What is justice? and Is the just or unjust life better for a person? In the process of answering these questions, he defends a sublime theory of the nature of reality and human knowledge. The ultimate foundation of Plato’s metaphysics—his view of reality—is his theory of Forms, culminating in the Form of the Good. Few, says Plato, really understand the nature of the Good itself (505e).
1. The Sun Analogy
The Form of the Good sits atop Plato’s hierarchy of being as the ultimate Form. The Forms themselves are abstract, although they do inform the concrete world, and Plato frequently relies on metaphor to describe them. To understand the Good itself, Plato relies on an analogy with the sun.
There are visible objects, which are visible but not intelligible in themselves. (Plato’s central concern is that the world of material objects is shifting, deceptive, and unreliable.) Then there are the Forms themselves, which are intelligible but not visible (507b). The Form of the Good, Plato says, is to the intelligible realm as the sun is to the visible realm. In the visible realm, there is a need of “something else” to make things visible, namely, the sun (507d). We need sight in ourselves and color in objects, but we also need the sun, or light, to make those things really visible, detectable by us.
Sight receives its power to see from the sun, as if from an overflowing treasury. And sight is the most “sunlike” of the senses, i.e., it and the sun have a kind of affinity or compatibility. The sun and our sense of sight go together. Likewise, the intelligible realm receives its order and intelligibility from the Form of the Good. Without the Form of the Good, we would be like people fumbling in the dark, with a capacity to understand but no “third thing” to render the world intelligible.
Plato says that the sun is the “cause” of the visible realm. The connection here may seem tenuous, but this much is clear: without the sun, most or all of the things on the earth would die out. This is presumably what Plato means. In the intelligible realm, the Form of the Good plays the same role: it is not only the reason for the intelligibility of the Forms, but the source of their existence as well. Though, as Plato says, the existence that it enjoys is “beyond being, superior to it in rank and power” (509b).Plato’s mysticism—his conviction that there are superior forms of existence—inherited from Pythagoras, is here on full display.
2. A Divine Order
So, the Form of the Good is more real, even, than the rest of the Forms: the realest and most fundamental thing that exists, the cause of the Forms and the explanation of the rational order of the universe. It resembles a divine logos, or divine rationality, which became an object of worship for successive schools of philosophy that developed under the influence of Plato’s ideas.
Nowadays, we might compare the Form of the Good to laws of nature, though this is not fully satisfying, since the Form of the Good is not particular law of nature, but the reason why there are laws at all. Stephen Hawking famously quipped that we should ask not only what the equations governing the universe are, but also “what breathes fire into the equations?” For Plato, both the equations and the fire are the Form of the Good.
3. The Forms and Human History
We can gauge the significance of Plato’s contributions to humanity by his influence on the history of thought. Plato’s was the first major metaphysical system in the West, and it dominated Western thought through the middle of the second millennium.
Consider the subject of mathematics and geometry. What is a point? It is a location in space with no dimension. In other words, it is not a real object. Points are ideal entities, not space-time particulars. They take up no space. Likewise, lines have length but no breadth. Mathematics is about ideal entities, and some mathematicians today are still “Platonists” about numbers: they hold the view that numbers or other mathematical objects are immaterial things. And they have to be in order for us to be able to know eternal truths about them.
If we live in a rationally ordered cosmos, this helps underwrite a social order that is rigidly hierarchical. It is no surprise then that through the Middle ages humans organize themselves into strict hierarchies. We find a hierarchical church and a stratified social structure, with serfs serving the king and the king serving God.
Consider Plato’s influence on theology: The Form of the Good is the ground of all being, an immaterial object that exists more perfectly than anything else, a thing responsible for the goodness and rationality in the world. This is something like an interpretation of the Christian view of God developed in the Middle Ages, founded in Platonic and Neo-Platonic metaphysics.
Perhaps most importantly, Plato’s arguments in Republic make possible scientific inquiry. Science is only possible if the natural world is intelligible to our rational faculties. Many people credit Plato’s student Aristotle with the initiation of the scientific project of humanity, and many in turn credit the scientific method as the West’s most profound contribution to humanity.
 Republic is firstly an argument about the ideal structure of a city. Notoriously, Plato installs philosopher-kings as a benevolent council. If the rulers of the city are to make themselves, their citizens, and their city good, they must first know Goodness itself.
 For example, a concrete table is a table rather than, say, a chair or a dog, because it participates—albeit partially—in the Form of table-hood. Concreta are what they are because of the Forms they participate in, and the Forms are ontologically and explanatorily prior to concreta.
About the Author
Dr. Ryan Jenkins is an assistant professor of philosophy and a senior fellow at the Ethics + Emerging Sciences Group at California Polytechnic State University in San Luis Obispo. He studies the ethics of emerging technologies, especially automation, cyber war, autonomous weapons, and driverless cars. His work has appeared in journals such as Ethical Theory and Moral Practice, and the Journal of Military Ethics, as well as public fora including Slate and Forbes. http://calpoly.academia.edu/RyanJenkins