Logically, Socrates must at this juncture entertain a definition of Goodness, but we cannot accept the premise that "knowledge of the Good is Goodness"; that constitutes an invalid argument (a false tautology). And some people offer other invalid arguments for Goodness, as we may observe.
Socrates then says that he will not precisely define Goodness, but that he can elucidate the argument by arguing another analogy. Socrates' analogy involves a comparison between sight and knowledge. In order for men to see, men must be given visible objects to perceive, and men must be given light in order to perceive the objects. The source of this light is the sun. Analogically, in order for men to know anything, men must be able to think, and they must be provided objects of knowledge (the Forms). Visible objects, then, have to be in the light; objects of knowledge have to be true. Light comes from the sun; truth comes from Goodness. (This analogy has come to be known as the Analogy of the Sun.)
