Jared Blumenfeld , director of San Francisco's Department of the Environment, gives an effective defense of the Precautionary Principle in Monday's San Francisco Chronicle. He says that "environmental decision-making be based on rigorous science" But the alarm bells go off for nanotech advocates when he writes: "Unfortunately, in today's regulatory system, lack of proof of harm is usually misinterpreted as proof of safety." I'm not sure if that's a "misinterpretation," or simply the human urge to progress despite an element of risk. If we didn't have the instinct for taking risks, I'd be chiseling this message on a cave wall. The full commentary is on Howard Lovy's NanoBot.