Platonicae quaestiones

But observe whether this be not true, that no circle

or sphere in this world is exact; but since by the tension and circumtension of the right lines, or by the minuteness of the parts, the difference disappears, the figure seems circular and round. Therefore no corruptible body moves circularly, but altogether in a right line. To be truly spherical is not in a sensible body, but is the element of the soul and mind, to which he has given circular motion, as being agreeable to their nature.