Four cases: (1) Public-goods underprovision (defence, basic research, contagious-disease surveillance) — there is no activity to brake, only one to start; a Samuelson-style public-goods rule is needed for the production side. (2) Voluntary risky behaviour with social spillovers (smoking, motorcycling without a helmet, recreational drugs) — the user is internal; the emergency-room budget is external; the criterion says price the external portion, not ban the activity. (3) High-variance frontier activity (gain-of-function research, novel financial instruments) — tail risk to external parties can dominate even when the central estimate of ΔW_ext is positive. (4) Effects on the unborn — the criterion treats future generations as external parties represented by the discount rate, which involves an ethical convention people of good faith disagree about.