核心畫面

One way to hold this word: a blade gets thinner and thinner until it ends in a single fine line. There is almost nothing there — no width to stand on, no flat place to rest. Anything set on that line touches it along one thread and has nothing holding it to either side. Every use of knife-edge can be read off that picture: the line itself, the thing balanced on it, and the feeling of being the one up there.