Prajna, or discriminating awareness, is an ongoing process—constantly cutting, all the time. So all the time it is cutting, it has to sharpen its own blade. Otherwise, you would have to stop cutting to sharpen the blade. So it’s an inbuilt mechanism, so to speak. Cutting through sharpens itself, because it cuts itself as soon as it cuts the other.
From Illusion’s Game: The Life and Teaching of Naropa by Chögyam Trungpa, page 79