The principle that where an activity or technology threatens serious or irreversible harm, lack of full scientific certainty should not be a reason to postpone protective measures; strong versions place the burden of proof on proponents to show safety.
In more detail
The principle is central to green politics and appears in international environmental law and in European Union policy. Green parties have invoked it in opposing nuclear power and genetically modified organisms and in calling for regulation of new technologies, including AI. Weak versions permit precaution; strong versions demand proof of no significant harm. Critics argue that strong versions can block beneficial innovation and ignore the risks of inaction.