Sir

The so-called ‘precautionary principle’ (PP) has gained currency in discussions about environmental protection and genetic manipulation, but it should be treated with caution.

The principle has been endorsed in international treaties, including the consolidated version of the treaty establishing the European Union. In many of these documents the PP has not been explicitly defined, but the Wingspread conference attempted to define it1. We believe the following definition would be accepted by most proponents:

“When an activity raises threats of serious or irreversible harm to human health or the environment, precautionary measures that prevent the possibility of harm (for example, moratorium, prohibition) shall be taken even if the causal link between the activity and the possible harm has not been proven or the causal link is weak and the harm is unlikely to occur.”

In our view, there are problems with the PP as so defined. The PP tells us to balance evidence in a specific way. The weight given to evidence is ordinarily thought to be a function of its epistemic warrant (the degree to which we have reasons for believing the evidence). The PP instructs us to change this normal balancing by giving evidence pointing in one direction more importance than evidence pointing in the other direction, even in cases where the evidence has the same epistemic warrant. Such discounting will distort our beliefs about the world, and will lead us to hold false beliefs. The PP cannot therefore be a valid principle for evaluating evidence.

As a principle of rational choice, the PP will leave us paralysed. In the case of genetically modified (GM) plants, for example, the greatest uncertainty about their possible harmfulness existed before anybody had yet produced one. The PP would have instructed us not to proceed any further, and the data to show whether there are real risks would never have been produced. The same is true for every subsequent step in the process of introducing GM plants. The PP will tell us not to proceed, because there is some threat of harm that cannot be conclusively ruled out, based on evidence from the preceding step. The PP will block the development of any technology if there is the slightest theoretical possibility of harm. So it cannot be a valid rule for rational decisions.

This fatal weakness of the PP illustrates a common problem in attempting to convert moral choices into legislation. The temptation is great to try to find one absolute and easily applicable principle, but such a principle will often be simplistic and will, when applied, lead to unjustifiable conclusions. Many moral choices are complex, and in making political decisions we should not lose sight of this complexity.