Full Idea
If the earth is spherical and in the middle of the heavens, it needs neither air nor force to keep it from falling. The uniformity of heaven and equilibrium of earth are sufficient support.
Gist of Idea
If the Earth is spherical and in the centre, it is kept in place by universal symmetry, not by force
Source
Plato (Phaedo [c.382 BCE], 108e)
Book Reference
Plato: 'The Last Days of Socrates', ed/tr. Tredennick,Hugh [Penguin 1969], p.171