The precautionary principle is invoked in circumstances where it cannot do the work required of it, and this has damaged a defensible idea. In its strong form — that an activity should not proceed until it is shown to be safe — it is incoherent as a general rule, since refraining is itself an action with consequences that are equally unproven, and applying the principle to the alternatives blocks everything including inaction. In its weak form — that the absence of conclusive evidence of harm is not evidence of its absence, and that regulators may act on well-grounded suspicion — it is unobjectionable and close to trivial. Most disputes conducted in its name are disputes about where between these poles a particular case falls, conducted in language that suggests a principle is being applied when a judgement is being made.