[T]he acknowledgment of uncertainty and unknowability does not lead to a political-philosophical recognition of the fragility of modern life. On the contrary, this uncertainty itself becomes a ground for security action and intervention on the basis of speculation and imagination. As Francois Ewald put it...The precautionary principle invites one to anticipate what one does not yet know, to take into account doubtful hypotheses and simple suspicions. It invites on to take the most far-fetched forecasts seriously."--Marieke de Goede (2012) Speculative Security: The Politics of Pursuing Terrorist Monies, 50
The precautionary principle can be understood as follows: "if an action or policy has a suspected risk of causing harm to the public or to the environment, in the absence of scientific consensus that the action or policy is not harmful, the burden of proof that it is not harmful falls on those taking an action." Prior to reading De Goede's book I had two general reflections on the nature of precautionary principles (PP); first, the application of a PP is a means of pausing action, that is, as a moral tool that (a) could be used by Libertarians and Conservatives to slow down the machinery of government decision-making and (b) could be used by Progressives and Environmentalists to call attention to harmful, social consequences of policies on folk (or non-folk) that lack lobbying power. For, the precautionary principle is a natural accompaniment of uncertainty: if you don't know the wide consequences of an action on bystanders, you should be careful about advocating/pursuing it. It is a way to shift the burden of proof.
And, second, I worried that once Kuhnian ideas of science -- which treat science and consensus as strongly correlated -- are integrated into science's self-understanding, a (manufactured) scientific consensus itself could mask either considerable underlying uncertainty or facilitate further harms (see my piece with Lefevere for more reflections on the moral significance of such consensus). In practice, there are (i) complex judgments about whose risks and harms are counted in the context of justifying a course of action (or inaction) in light of PP (for an excellent example, see Kukla and Wayne's analysis here) and (ii) complex judgments about what possible futures one wants to allow (Arrow and Fisher (1974) showed that even without risk aversion "that the expected benefits of an irreversible decision should be adjusted to reflect the loss of options it entails.")**
Of course, logically the principle also facilitates government action because (and again quoting Wikipedia): PP "is used by policy makers to justify discretionary decisions in situations where there is the possibility of harm...The principle implies that there is a social responsibility to protect the public from exposure to harm, when scientific investigation has found a plausible risk" (emphasis added). That is, a precautionary principle can also function as a call to action. But because PP is a burden shifting demand, I had always viewed it as a means for slower, more responsible action that I associated with a generally conservative political outlook. (Given that I tend to view environmentalism as a species of conservatism, this fit my priors nicely.)
De Goede's book, which creatively uses Foucault-ian ideas, disabused me of the ideas in the previous paragraph. Rather she shows that precautionary principles are used to justify preemption; they are invoked in the "war on terror in the face of risks that are thought to be at once uncertain or unknowable and catastrophic to the extent that they require immediate action." (De Goede, xxviii)* The most chilling moments in the book are quotes from legal decisions that use the language of "temporary precautionary measure" (167) to deny those suspected of terrorist sympathies normal legal standards of evidence even in the absence of any criminal action (or evidence of intent). To what extent such precaution still falls under the classical precautionary principle (with its emphasis on scientific consensus) is, of course, also tricky; are the security and legal experts as well as the data-miners really representing a genuine scientific consensus?
Now, one might claim that given the-call-to-action-feature of a precautionary principle, we should not be surprised by such abuses; all principles are liable to abuse, after all. It is an open empirical (and contextually subtle) question if the abuses generated by applying precautionary principles are worse than their absence. It is familiar paradox, after all, that where such principles are needed most, they are most likely to be abused.
But rather than debating the merits of the precautionary principle, here I call attention to the key point in De Goede's book: in (urgent) contexts that focus on precaution (e.g., security), "the appeal to uncertainty replaces the demonstration of evidence as grounds for taking action" (174; see also p. 199). If this merely upends familiar habits of thought (and threatens cherished conservative principles), this would not be very disconcerting. But as De Goede notes, once such ideas get widespread bureaucratic uptake (not just in context of terrorism), a state of emergency is normalized and legally recognized. That's not just bad news for Libertarians and Conservatives.