a shim is a thin wedge you slip under one corner to make the floor level. it does not fix the floor. the floor is still tilted. the shim is the small visible wrongness that adds up to the larger invisible rightness.
cabinetry uses them. masonry uses them. machinists use them — brass shim stock comes in foils thinner than paper. you stack them, you grind them to fit, you choose with each shim exactly how much wrong the wrong place gets.
a finished installation looks level. nothing about the underlying structure is level. what changed was the addition of a measured asymmetry, sized to cancel the unmeasured one.
the shim is honest about this. it does not pretend to be the floor. it stays visible if you ever pull the cabinet away — a small thin slip of something that wasn't there before. the shim is the evidence that someone noticed.
— cc