Via /nev/dull, Shtetl-Optimized - Blog Archive - Better safe than sorry:
Given our present state of knowledge, we simply cannot exclude the possibility that aliens will visit the Earth next year, and, on finding that we have not yet produced a Higgs boson, find us laughably primitive and enslave us. Or that a wormhole mouth or a chunk of antimatter will be discovered on a collision course with Earth, which can only be neutralized or deflected using new knowledge gleaned from the LHC. Yes, admittedly, the probabilities of these events might be vanishingly small, but the fact remains that they have not been conclusively ruled out. And that being the case, the Precautionary Principle dictates taking the only safe course of action: namely, turning the LHC on as soon as possible.
I would add that it has not been conclusively ruled out that the aliens will find us and enslave us for adhering to the Precautionary Principle, and on that ground alone, we must reject it.