Iron sharpens iron . . . but not always and not by default. Pieces of iron remaining still, side by side, or moving together in the same direction will not increase in sharpness, nor will pieces merely pressed against each other or struck together arbitrarily or without measure. This tends to result in dullness.

The sharpening process requires force, friction, heat – violence of a sort – but also precision and care. One imagines that for the iron itself it is not an easy or painless process. To be sharpened, a piece must be reshaped, remolded; it must give up part of what it once was and, at least in a small way, be changed into something new.