Allegory of the Cave
Socrates elaborates upon the immediately preceding Analogies of the Sun and of the Divided Line in the Allegory of the Cave, in which he insists that the psyche must be freed from bondage to the visible/sensible world by making the painful journey into the intelligible world. He continues in the rest of this book by further elaborating upon the curriculum which a would-be philosopher-king must study. This is the origin of the quadrivium: arithmetic, geometry, astronomy, and music.