Plato, with his belief that the world was constructed with geometric simplicity and elegance, felt certain that the sun, moon and planets would have a natural circular motion, since that is the simplest uniform motion that repeats itself endlessly, as their motion did. 

However, although the “fixed stars” did in fact move in simple circles about the North star, the sun, moon and planets traced out much more complicated paths across the sky. 

These paths had been followed closely and recorded since early Babylonian civilization, so were very well known.  Plato suggested that perhaps these complicated paths were actually combinations of simple circular motions

Plato believed the planets to be animate beings.  He argued that it was not possible that they should accurately describe their orbits year after year if they didn’t know what they were doing—that is, if they had no soul attached.