The Principles of Plato

Plato makes an analogy between what sun means to the visible realm and what the form of good means to the intelligible realm of forms (508a). First, just as the light of the sun makes the visible realm apparent to the eye, so it is by the light of form of good that the nature of reality is made apprehensible to the soul. Second, thanks to the sun the eye can see. Similarly, the form of good gives us the ability to have knowledge. Lastly, the sun causes things to exist or happen to be in the visible realm. For example, it regulates seasons. It is the cause of nourishment and generation. In the same way, the form of good is responsible for the existence of all other forms. Thus, Socrates says that “the form of good is beyond being; it is cause of all existence” (507b).

However, until the very next metaphor, he does not explicitly show how important this form of the good is to knowledge. This character of the divided line is something by which we can distinguish it from the previous imagery of sun. The analogy of the line is intended to illustrate the degrees of accessing the world, the levels of knowledge and opinion available to us (509d – 511b). Socrates provides us a line that is broken into four segments. A line is cut into two unequal parts, and each of them is divided again in the same proportion. The two main divisions represent the intelligible world and the visible world. The segment at the bottom includes imagination as the lowest grade of cognitive activity, plus a higher stage on this segment that is belief. In the imagination section, we have shadows or reflections of sensible objects. Belief or opinion is also the realm of the visible, but it contacts perceptible objects. In the belief, we have 6 humans, animals, plants, artifacts. A human being in this segment of belief thinks that sensible particulars are the most real things in the world. Further up the line, top two segments (intelligible division) consist of thought and understanding. Although thought deals with forms, it needs sensible particulars and hypotheses, like when in geometry we use a picture of a triangle to reason triangularity, or make appeal to axioms or hypothesis to prove theorems.

The other, higher section in the intelligible division also the highest and largest segment in the line also consists of forms but is accessed by understanding. But understanding does not require axioms or hypothesis in reasoning. This is why thought is inferior to understanding. The reasoning in understanding deals exclusively with forms, working with the unhypothetical first principle of everything which is the form of the good. It is possible to interpret the reasoning in understanding in terms of dialectic. Through dialectic, philosopher would question assumptions and arrives ultimately to an unhypothetical first principle of everything that is the form of the good. As we see, the divided line operates with images that were absent in sun allegory.