Such methods aim to offer robust and objective aids to decision-making. Precaution, by contrast, seems rather ambiguous and impractical. For some, precaution threatens even to undermine science by opening the floodgates to subjectivity. The answers to these questions are well established but surprisingly neglected. First, there's the issue of robustness.
Risk assessment requires at least some basis for assigning probabilities.
Under conditions of uncertainty, however, this can be obstructed by the novelty or complexity of the processes involved. Under even less tractable conditions of ignorance, some possibilities themselves may be unknown. By definition, risk assessment is therefore inapplicable under some important and pervasive characteristics in environmental decision making. To pretend otherwise is quite simply unscientific. Then there's the issue of objectivity.