Plato in his book, The Republic is asked what he means by the “highest kind of knowledge, which the Guardians must attain is the knowledge of the Form of the Good”
Plato does this in three connected stories
The story of the sun
The simile of the divided line
The allegory of the cave
In the story of the sun Plato explains what he means by the highest kind of knowledge – the Form of the Good.
It is from the Good, he says that right acts and everything else derive their usefulness and value.
He rejects the idea that the Good might be pleasure or knowledge.
What the Good is, is clearly a matter of dispute, yet it is the end of all human endeavour.
He explains the Good in terms of the sun.
The sun, through its light enables the eye to see and the object to be seen. The sun then, says Plato is the offspring of the Good, occupying in the visible world a position which is similar to that of the Form of the Good in the world of Forms
VISIBLE WORLD INTELLIGIBLE WORLD
SunForm of the Good
Objects of sightObjects of knowledge (The Forms)
Just as then in the visible world the sun is the cause of light which enables visible things to be seen and of sight which enables the eye to see, though itself is neither light nor sight,
So in the intelligible world the Form of the Good is the cause of truth, which enables the Forms to be known and of knowledge which enables the mind to know.