The precautionary principle is just a coherent statement of what really is just common sense.
Here’s a partial definition, taken from the Wingspread Statement, the original statement of the precautionary principle:
When an activity raises threats of harm to human health or the environment, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically.
In this context the proponent of an activity, rather than the public, should bear the burden of proof.
The process of applying the Precautionary Principle must be open, informed and democratic and must include potentially affected parties. It must also involve an examination of the full range of alternatives, including no action.
Instead of asking the basic risk-assessment question—”How much harm is allowable?”—the precautionary approach asks, “How little harm is possible?”
In sum: Faced with reasonable suspicion of harm, the precautionary approach urges a full evaluation of available alternatives for the purpose of preventing or minimizing harm.