"Critics of the precautionary principle charge that it is self-defeating: by demanding we forgo any action whose harms cannot be ruled out, it would forbid the very precautions it recommends, since those too carry unruled-out harms. The charge has force only against a caricature. A defensible precautionary principle is not a rule about acts in isolation but about the comparison of options under uncertainty; it counsels favoring the option whose worst credible outcome is least bad. So understood, it cannot paralyze, for inaction is itself an option to be ranked, and refusing a precaution is not exempt from the same scrutiny it imposes on action. The critics' objection succeeds, in short, only by assuming the asymmetry—action scrutinized, inaction privileged—that the principle exists to deny."