The eye, Plato says,is unusual among the sense organs in that it needs a medium,namely light,in order to operate.The strongest and best source of light is the sun; with it, objects can be discerned clearly.Analogous things can be said of intelligible objects (i.e., the fixed and eternal forms that are the ultimate objects of scientific and philosophical study. 

“太阳隐喻”（Metaphor of the sun）：Plato uses the sun as a metaphor for the source of "illumination",arguably intellectual illumination,which he held to be The Form of the Good, which is sometimes interpreted as Plato's notion of God.The metaphor is about the nature of ultimate reality and how knowledge is acquired concerning it.

链接：太阳和洞穴：灵魂由黑暗到光明