Eidetic variation: finding the essential

To grasp what is essential to something, a friendship, a promise, justice, Husserl varied it freely in imagination and watched which changes destroyed the thing and which left it intact. The features you can drop without losing it are accidental; the one whose removal collapses the thing is the invariant, the essence you were after. Today it is a sharp tool for thinking: it cuts through vague definitions and shows what a concept cannot do without.