The precautionary principle comes up a lot when you're talking about the side effects of technology in the real world. When you don't have evidence that something is dangerous — but you suspect it might be — you could cite the precautionary principle as a reason to ban or limit the use of that thing. It's a messy idea, though, and I'm still not sure what to think about it. On the one hand, technology is often available before data on the wide-ranging effects of that technology are available. Do you use it or not is a legitimate question. On the other hand, following the precautionary principle in a blind sort of way can lead to things like this.