My guess: Sharpening is the process of removing material, if you sharpen a knife forever it will grind down to a useless stub. It is possible something got lost

My guess: Sharpening is the process of removing material, if you sharpen a knife forever it will grind down to a useless stub. It is possible something got lost in translation, or possible that the metaphor isn't quite as satisfying as some of the others used.