Sounds obvious, eh? And yet there are so many people for whom this utterly correct thought means that you still act as if what you don't know is true. That, of course is the opposite of the principle above, but I recognize that the precautionary principle is an example of bad science and worse thinking that people engage in all the time.
The moral of the story is that there are always things to fear for those who wish to be afraid. For those who wish to do good in the world, fear is not your friend.