In Book VI of Plato’s Republic, Socrates explains why people reproach philosophers as useless and evil. He draws an analogy between a ship with a mutinous crew and a society with rebellious citizens. The mutinous crew members of a ship violently struggle with one another to become captain, but not one of them possesses knowledge of navigation. The crew considers the captain, who does possess such wisdom, a “star-gazer” because the crew does not realize that the constellations provide an excellent guide to navigate the ocean. Just as the crew’s ignorance of navigation causes them to unjustly mutiny against the captain, so too does the citizens’ ignorance of statesmanship cause them to rebel against true philosophers.
Next, Socrates explains why there are no philosopher-kings. He argues that societies are not conducive to the development of philosophers. Public opinion corrupts the majority of citizens. For example, when family and friends recognize the unique intellectual talents of an individual, they persuade him to pursue an occupation that is financially profitable. Whether the occupation leads to the acquirement of virtue is irrelevant. Because public opinion is very persuasive, Socrates advises that potential philosopher-kings ought to endure a rigorous physical and intellectual education, which consummates in the understanding of the Form of the Good.
To clarify the concept of the “Form of the Good,” Socrates uses a simile, which is now known in academia as the Simile of the Line. In the Simile of the Line, Socrates divides reality into two realms – the physical realm and the intelligible realm. In the physical realm, there are objects and there are images of those objects. For example, a tree is an object within the physical realm, and the reflection of that tree in a lake is an image within the physical realm. Just as two orders of ideas exist in the physical realm, so too do two orders of ideas exist in the intelligible realm. The lower order of ideas in the intelligible realm is composed of definitions of the Forms. For example, the definitions of Beauty, Courage, and Justice belong to this category. The higher order of the intelligible realm is composed of the Forms themselves – i.e. Beauty itself, Courage itself, Justice itself, etc.
In the physical realm, the sun holds a notable position. It shines its light on all objects, and thus enables people and other creatures to see these objects. It also gives existence to all things. Plants and animals could not exist without the sun. In the intelligible realm, the Form of the Good acts like the sun. It renders the definitions of the Forms and the Forms themselves intelligible, and it also gives them existence.
Socrates divides reality into the physical realm and the intelligible realm because he believes that abstract concepts exist independently of physical phenomena. Because abstract concepts do not exist in the physical realm (we do not see the number two hanging from a tree), they must exist in an intelligible realm. But other philosophers have challenged Socrates’ assumption that abstract concepts exist independently form physical phenomena. For example, the empiricist philosophers – such as John Locke and David Hume – believe that individuals derive abstract concepts from physical phenomena. They argue that individuals observe several different trees and abstract particular characteristics from those trees until only the shared qualities of the several trees remain. These shared qualities form the abstract idea of a tree. Without the several physical instances of trees, the abstract concept of a tree would not exist. Who is right – Socrates or the Empiricists?