Why does Mind go in circles? Why does Soul go in straight lines? What does either even refer to?
For starters, circular motion is perfect (in the literal sense of "complete"): when a circle revolves about its center, there's nowhere that it goes to that it wasn't already, and nowhere that it used to be that it isn't anymore. In the Timaeus, Plato calls the circular motion of the heavens "the moving image of eternity"—where "image" is in the sense of an icon, a copy, etc.
Contrast that with rectilinear (straight-line) motion, which lacks that sort of perfection, always going from one thing to another.
Circular motion, therefore, is the middle term between the unmoving/unchanging (eternity, ever complete in itself), and that which changes in a way that involves incompleteness (rectilinear motion).
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For starters, circular motion is perfect (in the literal sense of "complete"): when a circle revolves about its center, there's nowhere that it goes to that it wasn't already, and nowhere that it used to be that it isn't anymore. In the Timaeus, Plato calls the circular motion of the heavens "the moving image of eternity"—where "image" is in the sense of an icon, a copy, etc.
Contrast that with rectilinear (straight-line) motion, which lacks that sort of perfection, always going from one thing to another.
Circular motion, therefore, is the middle term between the unmoving/unchanging (eternity, ever complete in itself), and that which changes in a way that involves incompleteness (rectilinear motion).