The ‘precautionary principle’ is the idea that “scientists, medical researchers, technologists and just about everybody else these days should err on the side of caution lest they cause harm to human health or the environment.” But one of Britain’s leading medical experts says the principle inhibits knowledge. “He wonders whether, if the precautionary principle had been about for the past 200 years rather than the past 20, breakthroughs such as blood transfusions would ever have been made.”