A Metaphor

You know how knife sharpening works? You have a wheel, solid and rough, studded with tiny stones, that rotates at an almost unimaginably fast speed. You take the knife, and you bring the knife up against the wheel like so, and the wheel spins and grinds, ripping away bits of the knife as it removes everything besides that focused edge.

This is like that, only the knife that you must hold against the wheel is yourself.