Full Idea
As goodness stands in the intelligible realm to intelligence and the things we know, so in the visible realm the sun stands to sight and the things we see.
Gist of Idea
Good has the same role in the world of knowledge as the sun has in the physical world
Source
Plato (The Republic [c.371 BCE], 508c)
Book Reference
Plato: 'Republic', ed/tr. Waterfield,Robin [OUP 1993], p.235
A Reaction
The claim seems to be that only goodness makes the world intelligible, but that strikes as closer to mysticism than to objective observation.