The Precautionary Principle, as described by Fenton Robb (Letters, 26 September) is flawed because it involves trying to prove a negative, something that is impossible.
For example, it is not possible to prove that GM crops are not harmful. Every innovation involves risk, but that has not stopped the advance of technology. For example, it has not stopped the continued development of diesel engines, even though we know that their emissions are harmful.
The guiding principle ought to be that the innovation does no harm, or at least very little harm, compared with the benefit (this is assessed by cost-benefit analysis).
It should be allowed to proceed if, in its implementation, no evidence, or very little evidence, is found that it is harmful.